0038_P451_Sort_Characters_By_Frequency.cpp

/* 
Day: 38
Problem Number:  451 (https://leetcode.com/problems/sort-characters-by-frequency/)
Date: 07-02-2024 
Description:
Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string.
Return the sorted string. If there are multiple answers, return any of them.
 
Example 1:
Input: s = "tree"
Output: "eert"
Explanation: 'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:
Input: s = "cccaaa"
Output: "aaaccc"
Explanation: Both 'c' and 'a' appear three times, so both "cccaaa" and "aaaccc" are valid answers.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:
Input: s = "Aabb"
Output: "bbAa"
Explanation: "bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.
 
Constraints:
* 1 <= s.length <= 5 * 105
* s consists of uppercase and lowercase English letters and digits.

Code:  */
class Solution {
public:
    string frequencySort(string s) {
        unordered_map<char, int> cnt;
        for (char& c : s) {
            ++cnt[c];
        }
        vector<char> cs;
        for (auto& [c, _] : cnt) {
            cs.push_back(c);
        }
        sort(cs.begin(), cs.end(), [&](char& a, char& b) {
            return cnt[a] > cnt[b];
        });
        string ans;
        for (char& c : cs) {
            ans += string(cnt[c], c);
        }
        return ans;
    }
};