My Progress: 552

In this repo, I do not list those common and easy problems.
One problem may be classified as multiple types.
(PS. 这个readme是用来骗提交的)

Weekly Mock

#	title	Name	solved	# Week
509	Fibonacci Number	Recursive	Yes	Week1
70	Climibing stages	DP	Yes	Week1
62	Unique Paths	DP	Yes	Week1
63	Unique Paths II	DP	Yes	Week1
120	Triangle	DP	Yes	Week1
279	Perfect Squares	DP	Yes	Week1
375	Guess Number Higher or Lower II	DP	!	Week2
312	Burst Balloons	DP	!(hard)	Week2
300	longest increasing subsequence	Binary Search O(nlong)	!	Week2
673	Number of Longest Increasing Subsequence	DP	two DPs	Week2
322	Coin Change	DP	special cases	Week2

Biweekly Contest 34

#	title	Level	solved
1588	Sum of All Odd Length Subarrays	easy	Array
1589	Maximum Sum Obtained of Any Permutation	medium	Array & greddy
1590	Make Sum Divisible by P	medium	Hashtable

Biweekly Contest 34

#	title	Level	solved
1572	Matrix Diagonal Sum	easy	Array
1573	Number of Ways to Split a String	medium	Greedy
1574	Shortest Subarray to be Removed to Make Array Sorted	medium	Monotic Array
1575	Count All Possible Routes	hard	DP

Biweekly Contest 33

#	title	Level	solved
1556	Thousand Separator	easy	String
1557	Minimum Number of Vertices to Reach All Nodes	medium	Graph
1558	Minimum Numbers of Function Calls to Make Target Array	medium	Math
1559	Detect Cycles in 2D Grid	hard	BFS

Biweekly Contest 32

#	title	Level	solved
1539	Kth Missing Positive Number	easy	Array
1540	Can Convert String in K Moves	medium	Math
1541	Minimum Insertions to Balance a Parentheses String	medium	Stack
1542	Find Longest Awesome Substring	Hard	Bit Mask

Biweekly Contest 31

#	title	Level	solved
1523	Count Odd Numbers in an Interval Range	easy	Math
1524	Number of Sub-arrays With Odd Sum	medium	DP
1525	Number of Good Ways to Split a String	Hashmap
1526	Minimum Number of Increments on Subarrays to Form a Target Array	hard	Difference array

Biweekly Contest 30

#	title	Level	solved
1507	Reformat Date	easy	String
1508	Range Sum of Sorted Subarray Sums	medium	Heap
1509	Minimum Difference Between Largest and Smallest Value in Three Moves	medium	Array
1510	Stone Game IV	medium	DP

Biweekly Contest 29

#	title	Level	solved
1491	Average Salary Excluding the Minimum and Maximum Salary	easy	Array
1492	The kth Factor of n	medium	Array
1493	Longest Subarray of 1's After Deleting One Element	meidum	Sliding Window(zero count)
1494	Parallel Courses II	hard	Graph(inverse thinking, start from the end)

Biweekly Contest 28

#	title	Level	solved
1475	Final Prices With a Special Discount in a Shop	easy	Array
1476	Subrectangle Queries	meidum	Array
1477	Find Two Non-overlapping Sub-arrays Each With Target Sum	medium	Prefix sum or Sliding window
1478	Allocate Mailboxes	hard	Build cost array(math problem), build dp array

Biweekly Contest 27

#	title	Level	solved
1460	Make Two Arrays Equal by Reversing Sub-arrays	easy	Array
1461	Check If a String Contains All Binary Codes of Size K	medium	String
1462	Course Schedule IV	medium	DFS
1463	Cherry Pickup II	hard	(bottom-top)DP+DFS

Biweekly Contest 26

#	title	Level	solved
1446	Consecutive Characters	easy	Sliding Window
1447	Simplified Fractions	medium	String
1448	Count Good Nodes in Binary Tree	medium	Top-down dfs
1449	Form Largest Integer With Digits That Add up to Target	hard	DP

Biweekly Contest 25

#	title	Level	solved
1431	Kids With the Greatest Number of Candies	easy	Array
1432	Max Difference You Can Get From Changing an Integer	medium	String
1433	Check If a String Can Break Another String	medium	String
1434	Number of Ways to Wear Different Hats to Each Other	hard	DP

Biweekly Contest 24

#	title	Level	solved
1413	Minimum Value to Get Positive Step by Step Sum	easy	Prefix Sum
1414	Find the Minimum Number of Fibonacci Numbers Whose Sum Is K	medium	greedy
1415	The k-th Lexicographical String of All Happy Strings of Length n	medium	recursive
1416	Restore The Array	hard	DP

Biweekly Contest 23

#	title	Level	solved
1399	Count Largest Group	easy	Yes(sort and count)
1400	Construct K Palindrome Strings	medium	Yes(Math, count odd-occurence numbers)
1401	Circle and Rectangle Overlapping	medium	Yes(Math, find closest points on edge)
1402	Reducing Dishes	hard	Yes(Sort)

Biweekly Contest 22

#	title	Level	solved
1385	Find the Distance Value Between Two Arrays	easy	Array
1386	Cinema Seat Allocation	medium	Array
1387	Sort Integers by The Power Value	medium	Sort
1388	Pizza With 3n Slices	hard	didn't solve it!!!

Weekly Contest 206

#	title	Level	solved
1582	Special Positions in a Binary Matrix	easy	Array
1583	Count Unhappy Friends	medium	Dict
1584	Min Cost to Connect All Points	medium	Greedy
1585	Check If String Is Transformable With Substring Sort Operations	hard	Deque

Weekly Contest 205

#	title	Level	solved
1576	Replace All 's to Avoid Consecutive Repeating Characters	easy	Set
1577	Number of Ways Where Square of Number Is Equal to Product of Two Numbers	medium	Dict
1578	Minimum Deletion Cost to Avoid Repeating Letters	medium	Array
1579	Remove Max Number of Edges to Keep Graph Fully Traversable	hard	Union find

Weekly Contest 204

#	title	Level	solved
1566	Detect Pattern of Length M Repeated K or More Times	easy	Array
1567	Maximum Length of Subarray With Positive Product	medium	Array(use two auxiliary arrays: pos and num)
1568	Minimum Number of Days to Disconnect Island	medium	DFS(Tricky: at most 2 days)
1569	Number of Ways to Reorder Array to Get Same BST	hard	Recursive(use combination)

Weekly Contest 203

#	title	Level	solved
1560	Most Visited Sector in a Circular Track	easy	Array
1561	Maximum Number of Coins You Can Get	medium	Greedy
1562	Find Latest Group of Size M	meidum	Union find
1563	Stone Game V	hard	DP

Weekly Contest 202

#	title	Level	solved
1550	Three Consecutive Odds	easy	Array
1551	Minimum Operations to Make Array Equal	medium	Math
1552	Magnetic Force Between Two Balls	medium	Binary Search
1553	Minimum Number of Days to Eat N Oranges	hard	BFS or DP

Weekly Contest 201

#	title	Level	solved
1544	Make The String Great	easy	String
1545	Find Kth Bit in Nth Binary String	medium	Recursive based on Symmetry
1546	Maximum Number of Non-Overlapping Subarrays With Sum Equals Target	medium	Predix Sum&Hashmap
1547	Minimum Cost to Cut a Stick	hard	DP(same as 312 Burst Bolloons. Interval DP)

Weekly Contest 200

#	title	Level	solved
1534	Count Good Triplets	easy	BF
1535	Find the Winner of an Array Game	medium	Array
1536	Minimum Swaps to Arrange a Binary Grid	medium	Greedy
1537	Get the Maximum Score	hard	Two pointers

Weekly Contest 199

#	title	Level	solved
1528	Shuffle String	easy	String
1529	Bulb Switcher IV	medium	Array
1530	Number of Good Leaf Nodes Pairs	medium	DFS

Weekly Contest 198

#	title	Level	solved
1518	Water Bottles	easy	Math
1519	Number of Nodes in the Sub-Tree With the Same Label	medium	DFS
1520	Maximum Number of Non-Overlapping Substrings	medium	Minmax
1521	Find a Value of a Mysterious Function Closest to Target	hard	Set

Weekly Contest 197

#	title	Level	solved
1512	Number of Good Pairs	easy	Array
1513	Number of Substrings With Only 1s	medium	Sliding Window
1514	Path with Maximum Probability	medium	BFS
1515	Best Position for a Service Centre	hard	Binary Search

Weekly Contest 196

#	title	Level	solved
1502	Can Make Arithmetic Progression From Sequence	easy	Array
1503	Last Moment Before All Ants Fall Out of a Plank	medium	MAXMIN
1504	Count Submatrices With All Ones	medium	DP

Weekly Contest 195

#	title	Level	solved
1496	Path Crossing	easy	Hashmap
1497	Check If Array Pairs Are Divisible by k	medium	Array(similar to target sum)
1498	Number of Subsequences That Satisfy the Given Sum Condition	medium	Array(sort then two sum)
1499	Max Value of Equation	hard	Convert to yi-xi+yj+xj, given j find max of (yi-xi), MAX heap

Weekly Contest 194

#	title	Level	solved
1486	XOR Operation in an Array	easy	Array
1487	Making File Names Unique	meidum	Hashmap
1488	Avoid Flood in The City	medium	Heap and Hashmap

Weekly Contest 193

#	title	Level	solved
1480	Running Sum of 1d Array	easy	Array
1481	Least Number of Unique Integers after K Removals	medium	Heap
1482	Minimum Number of Days to Make m Bouquets	medium	Binary Search
1483	Kth Ancestor of a Tree Node	Hard

Weekly Contest 192

#	title	Level	solved
1470	Shuffle the Array	easy	Array
1471	The k Strongest Values in an Array	medium	Array
1472	Design Browser History	medium	Array
1473	Paint House III	hard	Top-down dfs+DP

Weekly Contest 191

#	title	Level	solved
1464	Maximum Product of Two Elements in an Array	easy	Array
1465	Maximum Area of a Piece of Cake After Horizontal and Vertical Cuts	medium	Array
1466	Reorder Routes to Make All Paths Lead to the City Zero	medium	DFS, contraints show that there will be not loops in the graph
1467	Probability of a Two Boxes Having The Same Number of Distinct Balls	hard	Did not solve!

Weekly Contest 190

#	title	Level	solved
1455	Check If a Word Occurs As a Prefix of Any Word in a Sentence	easy	String
1456	Maximum Number of Vowels in a Substring of Given Length	medium	Sliding window
1457	Pseudo-Palindromic Paths in a Binary Tree	medium	Tree, check palindrome and run dfs
1458	Max Dot Product of Two Subsequences	hard	Same as longest common subsequence(1143)

Weekly Contest 189

#	title	Level	solved
1450	Number of Students Doing Homework at a Given Time	easy	Array
1451	Rearrange Words in a Sentence	medium	Array
1452	People Whose List of Favorite Companies Is Not a Subset of Another List	medium	Set
1453	Maximum Number of Darts Inside of a Circular Dartboard	hard	Math, O(N^3) find pairs of points and define a circle

Weekly Contest 188

#	title	Level	solved
1441	Build an Array With Stack Operations	easy	Array
1442	Count Triplets That Can Form Two Arrays of Equal XOR	medium	Prefix Sum, Assume a == b, thus a ^ a = b ^ a, thus 0 = b ^ a
1443	Minimum Time to Collect All Apples in a Tree	medium	Bottom-up DFS

Weekly Contest 187

#	title	Level	solved
1436	Destination City	easy	Array
1437	Check If All 1's Are at Least Length K Places Away	medium	Array
1438	Longest Continuous Subarray With Absolute Diff Less Than or Equal to Limit	medium	Heap
1439	Find the Kth Smallest Sum of a Matrix With Sorted Rows	hard

Weekly Contest 186

#	title	Level	solved
1422	Maximum Score After Splitting a String	easy	Use counter
1423	Maximum Points You Can Obtain from Cards	medium	Use sliding window to find the sub-array with max sum
1424	Diagonal Traverse II	medium	Indexes on the diagonal have the same SUM

Weekly Contest 185

#	title	Level	solved
1417	Reformat The String	easy	String
1418	Display Table of Food Orders in a Restaurant	medium	dictionary
1419	Minimum Number of Frogs Croaking	medium	the solution is very tricky
1420	Build Array Where You Can Find The Maximum Exactly K Comparisons	hard	3D dp, not easy to come up with

Weekly Contest 184

#	title	Level	solved
1408	String Matching in an Array	easy	Array
1409	Queries on a Permutation With Key	medium	Array
1410	HTML Entity Parser	medium	String
1411	Number of Ways to Paint N × 3 Grid	hard	DP

Weekly Contest 183

#	title	Level	solved
1403	Minimum Subsequence in Non-Increasing Order	easy	Sort
1404	Number of Steps to Reduce a Number in Binary Representation to One	easy	Math
1405	Longest Happy String	medium	Greedy
1406	Stone Game III	hard	DP

Weekly Contest 182

#	title	Level	solved
1394	Find Lucky Integer in an Array	easy	Yes(Math)
1395	Count Number of Teams	medium	Yes(backtrack)
1396	Design Underground System	medium	Yes(collection.dictionary)
1397	Find All Good Strings	hard	No(several test cases passed)

Weekly Contest 181

#	title	Level	solved
1389	Create Target Array in the Given Order	easy	Yes(sort)
1390	Four Divisors	medium	Yes(Math)
1391	Check if There is a Valid Path in a Grid	medium	Yes(DFS, BFS)
1392	Longest Happy Prefix	Hard	Yes(sliding widnow)

Weekly Contest 180

#	title	Level	solved
1380	Lucky Numbers in a Matrix	easy	Yes
1381	Design a Stack With Increment Operation	Medium	Stack
1382	Balance a Binary Search Tree	Medium	contruct a Binary Search Tree
1383	Maximum Performance of a Team	hard	Priority Queue(close to 857)

Array

#	title	note
665	Non-decreasing Array	detect first decrease pair, analyze two possible cases
1002	Find Common Characters	One pass(though easy question, pretty tricky)
1200	Minimum Absolute Difference	Sort + hashmap
867	Transpose Matrix	A[i][j]==new_A[j][i]
1260	Shift 2D Grid	Array
888	Fair Candy Swap	Set, find target value in another set
1089	Duplicate Zeros	Array
896	Monotonic Array	Compare adjacent elements
122	Best Time to Buy and Sell Stock II	Always buy ant valley and sell at next peak, (good graph explaination)[https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/discuss/208241/Explanation-for-the-dummy-like-me.]
1013	Partition Array Into Three Parts With Equal Sum	From left to right, find the first subset with target sum, then the second...
121	Best Time to Buy and Sell Stock	Typical sliding window technique to solve this problem
746	Min Cost Climbing Stairs	Top is confusing, make the dp array length -> length(cost)+1
830	Positions of Large Groups	One pass
1232	Check If It Is a Straight Line	Use on point and compute the slope with the rest
1128	Number of Equivalent Domino Pairs	Hashtable, note: list type is not hashable, convert to string
1010	Pairs of Songs With Total Durations Divisible by 60	2 Sum question by doing mod operation on the given array good question
1018	Binary Prefix Divisible By 5	Bit manipulation
674	Longest Continuous Increasing Subsequence	One pas, find start and end
724	Find Pivot Index	Typical prefix sum
849	Maximize Distance to Closest Person	find left 1 and right 1, between these two are all one's
1346	Check If N and Its Double Exist	Similar to target sum(2 sum)
941	Valid Mountain Array	Solution is easy but many conditions need to be satisfied
581	Shortest Unsorted Continuous Subarray	Sort + two indexes

Math

#	title	solved
7	Reverse Integer	Yes
165	Comapre Version Numbers	Yes
66	Plus one	Yes
258	Add Digits	Yes
67	Add Binary	Yes
69	Sqrt(x)	Yes
367	Valid Perfect Square	Yes(bianry search)
397	Integer Replacement	Yes
204	Count Primes	Redo(time exceed)
1	Two Sum	Yes
15	Three Sum	Yes
231	Power of Two	Yes

Hashmap

#	title	solved
454	4Sum II	Get sum count in A,B
846	Hand of Straights	Two maps: Order dict and need map(for storing the next needed number)

Two Pointers

#	title	note
61	Rotate List	Two solutions:1. iterate k times or use two pointers
457	Circular Array Loop	Mark visited elements using numbers
632	Smallest Range Covering Elements from K Lists	1. pointers+heap 2. sliding window
828	Count Unique Characters of All Substrings of a Given String	Find the last two occurence indexes for each letter
76	Minimum Window Substring	Sliding window, tricks: use two additional variables: found and target

BST

#	title	solved
98	Validate Binary Search Tree	1. Recursive 2. inoder traverse
701	Insert into a Binary Search Tree	1. inorder then build tree 2. recursive(we can always put the new val in leaf node)
99	Recover Binary Search Tree	Inorder to find the swapped elements

Union Find

#	title	solved
959	Regions Cut By Slashes	This union find solution does not apply path compression when doing the union operation
684	Redundant Connection	Each time before union, check if two vertices are in the same cluster using find
547	Friend Circles	A simple solution using Union Find
1202	Smallest String With Swaps	Convert to graph, then union, then sort
990	Satisfiability of Equality Equations	First union all the '==' equations, then find all '!=' equations
765	Couples Holding Hands	Consider each couple as a node, find all the connected clusters, apply permutation theorem(N//2- (# of clusters))
128	Longest Consecutive Sequence	Using set to keep track of the visited numbers, use union to connect all consecutive elements
839	Similar String Groups	Union find is right but Python version is LTE

Graph

#	title	solved
310	Minimum Height Trees	start from leaves, delete and find new leaves until n <= 2
399	Evaluate Division	one test case is wrong???
149	Max Points on a Line	hard, brute search
207	Course Schedule	DFS + extra array for checking if cur node will form a loop
210	Course Schedule II	BFS + queue
1377	Frog Position After T Seconds	BFS + queue, remove neighbors that have been visited
133	Clone Graph	Queue + hashmap
138	Copy List with Random Pointer	Similar to 133, queue + hashmap
827	Making A Large Island	DFS replaces island with unique index
1162	As Far from Land as Possible	BFS updates 0 to 1 till all become 1s

Design

#	title	solved
146	LRU Cache	Use ordered dict
284	Peeking Iterator	Use a variable to store peek value
341	Flatten Nested List Iterator	Use stack to break down nested list
352	Data Stream as Disjoint Intervals	1. Binary search 2. Heap
303	Range Sum Query - Immutable	Prefix sum

Prefix Sum

#	title	solved
304	Range Sum Query 2D - Immutable	dp[i][j] stores the element sum from arr[i][j] to arr[r-1][c-1]
1124	Longest Well-Performing Interval	Convert to -1/+1 representation and search for subarry with positive sum

DP

#	title	solved
416	Partition Equal Subset Sum	DP or set sum
392	Is Subsequence	Iterate T and compare each value with s[0]
1277	Count Square Submatrices with All Ones	Find the number of submatrices ending at index i,j
338	Counting Bits	dp[i]=dp[i>>1]+i%2
1130	Minimum Cost Tree From Leaf Values	Greedy, always find the smallest one and its neighbor
931	Minimum Falling Path Sum	Same as seam carving algorithm
1143	Longest Common Subsequence	2D DP
1105	Filling Bookcase Shelves	Unusual DP problem, current optimal solution relies on several previous states
1027	Longest Arithmetic Sequence	Use dictionary
646	Maximum Length of Pair Chain	O(N^2) dp solution, O(nlogn)+O(n) Greedy
343	Integer Break	find as many 3 as possible
1155	Number of Dice Rolls With Target Sum	dp_{ij} means i dice can get sum j
813	Largest Sum of Averages	dp_{ki} means by dividing first i numbers into k groups, the max sum we can get. O(kn^2)
1024	Video Stitching	dp_{ij} -> min clips to form i-j
650	2 Keys Keyboard	My solution is to find all the factors given an N, then we can repeat the copy operation
740	Delete and Earn	Similar to house robber but need to do some conversion
688	Knight Probability in Chessboard	3D DP
1039	Minimum Score Triangulation of Polygon	2D DP, DP_{ij} means from i to j, the min triangle sum we can get
873	Length of Longest Fibonacci Subsequence	dp[i][j]: max len of seq ending with A[i],A[j], dp[k][i]+1 = dp[i][j]
486	Predict the Winner	Same as 877(Stone Game)
1139	Largest 1-Bordered Square	Create auxillary horizontal and vertical arrays
659	Split Array into Consecutive Subsequences	Maintain two hashmap for keeping track of the number frequence and next nums

DP

#	title	solved
42	Trapping Rain Water	Yes
139	Work Break	Yes
518	Coin Change	Yes
64	Minimum Path Sum	Yes
72	Edit Distance	Yes
221	Maximal Square	Yes
198	House Robber	Yes
213	House Robber 2	Yes
337	House Robber 3	Yes
85	Maximal Rectangle	No
91	Decode Ways	Yes
494	Target Sum	Yes(2 solutions: dp and collections)
877	Stone Game	2D DP, DP[i][j]: more stones one can get than another
53	Maximum Subarray	dp[i] means max subarray ending at index i
576	Out of Boundary Paths	3D DP
664	Strange Printer	DP with memory

Binary Search

#	title	solved
278	Fisrt Bad Version	Yes
35	Search Insert Position	Yes
33	Search in Rotated Sorted Array	Yes(smallest index is tricky!)
81	Search in Rotated Sorted Array II(duplicate numbers)	Yes(search smallest index in O(logn) time!)
153	Find Minimum in Rotated Sorted Array	Yes
154	Find Minimum in Rotated Sorted Array II	Yes
374	Guess Number Higher or Lower	Yes
34	Find First and Last Position of Element in Sorted Array	Yes(find most left and most right index)
354	Russian Doll Envelopes	Yes(reduce to one dimension, then apply LLS)
1283	Find the Smallest Divisor Given a Threshold	Normal bianry search
875	Koko Eating Bananas	Same as 1283
1011	Capacity To Ship Packages Within D Days	O(nlogn), left bound is max(A) and right bound is sum(A)
1292	Maximum Side Length of a Square with Sum Less than or Equal to Threshold	Array presum + binary search for side length
1300	Sum of Mutated Array Closest to Target	Careful about the left and right index, left=mid, right=mid
668	Kth Smallest Number in Multiplication Table	Find number counts in each row
1201	Ugly Number III	Union and intersection, basic math logic
410	Split Array Largest Sum	Find satisfying splits
315	Count of Smaller Numbers After Self	Bisect module used
1235	Maximum Profit in Job Scheduling	Bisect to find the inserted time slot
1157	Online Majority Element In Subarray	Store indexes in increasing order and use bisect find left and right inserted indexes, then apply early stop
1095	Find in Mountain Array	Binary search finds mountain index and binary search finds target
719	Find K-th Smallest Pair Distance	Binary search distance
878	Nth Magical Number	Least common multiplier

Matrix

#	title	solved
48	Rotate Image	Yes
54	Spiral Matrix	Yes
59	Sipral Matrix 2	No
329	Longest Increasing Path in a Matrix	Yes
378	Kth Smallest Element in a Sorted Matrix	Yes
74	Search a 2D Matrix	Yes
240	Search a 2D Matrix II	Yes

Sliding Window

#	title	solved
1004	Max Consecutive Ones III	Yes(left and right index)
209	Minimum Size Subarray Sum	Yes(left right index), similar to 1004
904	Fruit Into Baskets	Yes(maintain a hashmap for count)
992	Subarrays with K Different Integers	hard Yes(convert to AtMost problem)
930	Binary Subarrays With Sum	Yes(two ways: AtMost and HashMap)
1234	Replace the Substring for Balanced String	Yes(left > right, the return 0)
1248	Count Number of Nice Subarrays	1. AtMost strategy 2. count
1358	Number of Substrings Containing All Three Characters	count # of letter on the left
3	Longest Substring Without Repeating Characters	find the longest subarray end at right
567	Permutation in String	backtrack(TLE), use COUNT
424	Longest Repeating Character Replacement	left and right index + count
1208	Get Equal Substrings Within Budget	left+right index, record cur maxCount value
5	Longest Palindromic Substring	start from center, left-=1 rigth += 1
647	Palindromic Substrings	Close to problem 5, center expansion
927	Three Equal Parts	Starting from end, find a potential matching bianry reprsentation

Segment Tree

#	title	solved
307	Range Sum Query - Mutable	Build Tree-> Update Tree-> Query Sum(all recursively)

Sort

#	title	solved
88	Merge Sorted Array	Yes
922	Sort Array By Parity II	tow pointers(tricky)
1122	Relative Sort Array	Yes
976	Largest Perimeter Triangle	Yes(Try biggest)
23	Merge k Sorted Lists	hard

DFS

#	title	note
1376	Time Needed to Inform All Employees	time=max(manager's employees)+informTime(manager)
1319	Number of Operations to Make Network Connected	DFS finds connected graphs(all the nodes reachable)
1145	Binary Tree Coloring Game	Find connected nodes and run dfs on each node to find the total count
199	Binary Tree Right Side View	level order traverse + DFS
105	Construct Binary Tree from Preorder and Inorder Traversal	DFS solve left and right sub-tree
721	Accounts Merge	DFS or Union Find
934	Shortest Bridge	DFS+BFS, DFS for island deetction and BFS for searching shortest path
988	Smallest String Starting From Leaf	leaf is a node where left and right are both None
1028	Recover a Tree From Preorder Traversal	Good problem
332	Reconstruct Itinerary	REDO
109	Convert Sorted List to Binary Search Tree	Find mid point value as new root
114	Flatten Binary Tree to Linked List	Restore preorder of the tree, then build new tree
980	Unique Paths III	DFS+memo
1192	Critical Connections in a Network	Good question(Tarjan algorithm), REDO
897	Increasing Order Search Tree	Inorder traverse to get all the nodes
979	Distribute Coins in Binary Tree	Post-order traverse, dfs(node) returns how many coins should go from(to) node.video explain
785	Is Graph Bipartite	DFS or BFS+dictionary, BFS is easier to understand
399	Evaluate Division	Tricky, build graph and run dfs or bfs

BFS

#	title	note
116	Populating Next Right Pointers in Each Node	Level order traverse
117	Populating Next Right Pointers in Each Node II	Same as 116
864	Shortest Path to Get All Keys	Use 3-elements tuple to keep track of seen blocks
1284	Minimum Number of Flips to Convert Binary Matrix to Zero Matrix	Convert matrix to bin representation and BFS finds all the possible states
847	Shortest Path Visiting All Nodes	Use bit to keep track of visited nodes in seen(set)

DFS and BFS

#	title	solved
200	Number of island	Yes
130	surrounded regions	Yes
127	Word ladder	Yes
397	Integer Replacement	Yes
417	Pacific Atlantic Water Flow	Yes
733	Flood Fill	Yes
778	Swim in Rising Water	Yes
433	Minimum Genetic Mutation	Yes (close to 127)
909	Snakes and Ladders	Yes
1025	Divisor Game	Yes
464	Can I Win	Yes(solve sub-problem)
110	Balanced Bianry Tree	Yes(DFS)
1315	Sum of Nodes with Even-Valued Grandparent	Yes(loof for grands from current node)
104	Maximum Depth of Binary Tree	Yes
841	Keys and Rooms	Yes
695	Max Area of Island	Yes(dfs is tricky, set visited cells to 0's)
1254	Number of Closed Islands	Yes(cells can be reached by edge cells will be set to 1's(waster))
1034	Coloring A Boarder	Yes(check four directionally connected cells) redo!!!
872	Leaf-Similar Trees	Yes
559	Maximum Depth of N-ary Tree	Yes
547	Friend Circle	Yes(DFS+memo)
1020	Number of Enclaves	Yes(close to 695)
802	Find Eventual Safe States	Yes(good dfs problem, help to know the dfs order)
108	Convert Sorted Array to Binary Search Tree	Yes
994	Rotting Oranges	Yes(should change from 'easy' to 'medium')
513	Find Bottom Left Tree Value	Yes(level order search)
515	Find Largest Value in Each Tree Row	Yes(close to 513, level order search)
752	Open the Lock	Yes(BFS, to improve runtime, use deque)
542	01 Matrix	Yes(close to 752)
1091	Shortest Path in Binary Matrix	Yes(template for solving shortest path problem)
1306	Jump Game III	Queue

Stack

#	title	solved
155	Min Stack	Yes
232	Implementing Queue using Stack	Yes
225	Imppementing Stack using Queue	Yes
150	Evaluate Reverse Polish Notation	Yes
394	Decode String	Yes
224	Basic Calculator	Yes
1019	Next Greater Node In Linked List	good question, REDO
739	Daily Temperatures	Similar to 1019
901	Online Stock Span	stack stores (val, res)

Queue

#	title	solved
215	Kth Largetst element in an Array	Yes
347	TopK frequent element	Yes
20	Valid Parentheses	a valid case results in an empty queue
862	Shortest Subarray with Sum at Least K	Keep increasing prefix sum
1425	Constrained Subset Sum	Keep a decreasing queue
743	Network Delay Time	BFS + deque
239	Sliding Window Maximum	Typical monotonic queue problem

Backtracking

#	title	solved
78	Subsets	Yes
90	No Duplicate Subset	Yes
77	Combination	Yes
39	Combination Sum	Yes
40	Combination Sum 2	Yes
216	Combination Sum 3	Yes
377	Combination Sum 4	Yes
46	Permutation	Yes
47	Permutation 2	Yes
17	Letter Combinations of a Phone Number	Yes
357	Count Numbers with Unique Digits	Yes(but very slow, faster than 5%, check bt template)
131	Palindrome Partitioning	Yes(can use template, note: path(reference) vs path[:](value))
22	Generate Parentheses	Yes(very slow, better solution: DFS, DP)
1219	Path with Maximum Gold	Backtrack + BFS, good practice(template)
1079	Letter Tile Possibilities	follow template
1239	Maximum Length of a Concatenated String with Unique Characters	need to improve run time
784	Letter Case Permutation	two cases in backtrack: lower case and upper case
491	Increasing Subsequences	backtracking

String

#	title	solved
31	Next Permutation	Yes
60	Permutation Sequence	Yes
159	Longest substring with at most two distinct characters	159
8	String to Integer (atoi)	If-else practice
869	Reordered Power of 2	Find all the power of 2 numbers first

Tree

#	title	solved
144	Pre Order Tree	Yes
94	In Order Tree	Yes
145	Post Order Tree	Yes
102	Level order Tree	Yes
100	Same Tree	Yes
101	Symmetric Tree	Yes
226	Invert Tree	Yes
257	binary tree path	Yes
112	binary tree sum	Yes
113	binary tree sum2	Yes
129	Sum root to leaves	Yes
111	Minimum depth of a tree	Yes
1302	Deepest Leaves Sum	Yes(traverse each level)
1110	Delete Nodes And Return Forest	DFS is the key, remove node + set children to None
107	Binary Tree Level Order Traversal II	BFS for level traverse
530	Minimum Absolute Difference in BST	Inorder traverse ensures that we get a sorted result from a BST
437	Path Sum III	Two ways: 1. run dfs on each node 2. hashmap storing prefix sum
404	Sum of Left Leaves	BFS and DFS are provided
669	Trim a Binary Search Tree	DFS: if node.val < L, find right subtree; if node.val > R, find left subtree
1022	Sum of Root To Leaf Binary Numbers	DFS to reach the leaf nodes to get the root-to-leaf path
654	Maximum Binary Tree	DFS build root.left and root.right
1008	Construct Binary Search Tree from Preorder Traversal	DFS for find root.left and root.right
993	Cousins in Binary Tree	DFS finds depth and parent node
637	Average of Levels in Binary Tree	BFS for level traverse
1325	Delete Leaves With a Given Value	DFS check if node is leaf and node.val==target, then return None
543	Diameter of Binary Tree	Good Question, similar to 687. DFS for finding the longest path for left and right subtree
572	Subtree of Another Tree	DFS finds if left and right subtree are identical
889	Construct Binary Tree from Preorder and Postorder Traversal	The property of pre and post determines that we can split these two order into new subset and contruct left subtree and right subtree
1026	Maximum Difference Between Node and Ancestor	The definition of ancestor is important. We keep track of the min and max values along the path from root to leaf, then the max difference would be max-min
508	Most Frequent Subtree Sum	Bottom-up approach for getting all the subtree sums
1372	Longest ZigZag Path in a Binary Tree	The key is to analyse the basic unit structure. root->left-right, root->right-left.
958	Check Completeness of a Binary Tree	Level traverse and find the first None node, keep looking if valid nodes exisit after it.
96	Unique Binary Search Trees	DP solution The special property of BST determines that for root j==3, and node values range from 1 to 5, we have 2 left nodes and 2 right nodes.
297	Serialize and Deserialize Binary Tree	Preorder to serialize a tree and then dfs to find the left subtree and right subtree.in dfs, do not send a copy of data, keep the original
652	Find Duplicate Subtrees	Hashtable + subtree serialize
863	All Nodes Distance K in Binary Tree	Build graph and run dfs.
450	Delete Node in a BST	Two solutions provided: 1. inorder traverse+buildTree 2. DFS updates left and right nodes
662	Maximum Width of Binary Tree	Level order traverse + complete binary tree numbering
106	Construct Binary Tree from Inorder and Postorder Traversal	Similar to 889, find left nodes and right nodes subsets
971	Flip Binary Tree To Match Preorder Traversal	DFS, check current node and children
95	Unique Binary Search Trees II	Recursively build left and right subtrees
1339	Maximum Product of Splitted Binary Tree	Get top-down sum for each node and run dfs to break the tree into two
1367	Linked List in Binary Tree	Run dfs to detect whether starting at a node can find the pattern, meanwhile dfs stores the path it goes through
1123	Lowest Common Ancestor of Deepest Leaves	Bottom-up approach

Linked List

#	title	solved
206	Reverse Linked List	Yes(remember the order)
141	Linked List Cycle	Yes
142	Linked List Cycle II	fast and slow pointers, math problem, good blog
24	Swap Nodes in Paris	Yes(prev, a, b)
328	Odd Even Linked List	Yes(break down to 2 linked list)
237	Delete a Node in a Linked List	Yes
19	Remove nth Node	Yes
83	remove duplicate numbers in linked list	prev + cur
203	Remove Linked List Elements	Yes
2	Add Two Numbers	get sum first and then make a linked list
445	Add Two Numbers II	same as 2
21	Merge Two Sorted Lists	add the smaller one each time
1171	Remove Zero Sum Consecutive Nodes from Linked List	prefix sum is the key
160	Intersection of Two Linked Lists	if reach end, switch head
92	Reverse Linked List II	reverse first then concat three sub-link
82	Remove Duplicates from Sorted List II	prev+cur, dummy for first non-duplicated node
148	Sort List	medium
86	Partition List	medium, use prev1 and prev2 to make new list
147	Insertion Sort List	medium, prev.next = cur.next, insert cur.val into right place
725	Split Linked List in Parts	medium, first think: what is the length for each part?
143	Reorder List	medium, get two list, start from most left and most right

Heap

#	title	solved
621	Task Scheduler	Always process the most frequent jobs, good explaination here
373	Find K Pairs with Smallest Sums	The next smallest sum must be at either right or down directions.
1046	Last Stone Weight	Maintain a max heap
703	Kth Largest Element in a Stream	Maintain a min heap with fixed size k
973	K Closest Points to Origin	Maintain a max heap with size K, everytime, we pop the current largest value
451	Sort Characters By Frequency	Two solutions
692	Top K Frequent Words	Maintain min heap with size k
767	Reorganize String	Max heap to store most frequent letters
313	Super Ugly Number	Min heap, find values in the given primes closure
1054	Distant Barcodes	Same as 767, use prev variables to keep track of the previous number
264	Ugly Number II	Same as 313
787	Cheapest Flights Within K Stops	Both DFS and BFS LTE
295	Find Median from Data Stream	Impressive solution, use max and min heap to maintain half numbers, top numbers at eahc heap will be numbers in the middle
658	Find K Closest Elements	Min heap
4	Median of Two Sorted Arrays	Max and min heap
57	Insert Interval	Merge

Random

#	title	solved
384	Shuffle An Array	Yes
398	Random Pick Index	Yes
382	Linked List Random Node	Yes
380	Insert Delete GetRandom O(1)	Tes
381	Insert Delete GetRandom O(1) - Duplicates allowed	Yes