1. If input array is sorted then - Binary search OR Two pointers / sliding window

2. If asked for all permutations/subsets then - Backtracking

3. If given a tree then - DFS - BFS

4. If given a graph then - DFS - BFS

5. If given a linked list then - Two pointers

6. If recursion is banned then - Stack

7. If must solve in-place then - Swap corresponding values - Store one or more different values in the same pointer

8. If asked for maximum/minimum subarray/subset/options then - Dynamic programming

9. If asked for top/least K items then - Heap - QuickSelect

10. If asked for common strings then - Map - Trie

11. Else - Map/Set for O(1) time & O(n) space

12. Sort input for O(nlogn) time and O(1) space