451. Sort Characters By Frequency (sort map)

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"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:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:
"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.

Approach #1: c++.

class Solution {
public:
    string frequencySort(string s) {
        unordered_map<char, int> mp;
        string ans = "";
        for (int i = 0; i < s.length(); ++i) {
            mp[s[i]]++;
        }
        vector<pair<char, int>> v(mp.begin(), mp.end());
        sort(v.begin(), v.end(), cmp);
        for (int i = 0; i < v.size(); ++i) {
            while (v[i].second--) {
                ans += v[i].first;
            }
        }
        return ans;
    }

private:
    static bool cmp(pair<char, int> a, pair<char, int> b) {
        return a.second > b.second;
    }
};