Sort Characters By Frequency问题及解法

问题描述：

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

示例:

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.

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.

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.

问题分析：

统计每个字符出现的次数，把出现次数相同的字符组成字符串存放到一起，最后按照出现次数由大到小组合起来即可。

过程详见代码：

class Solution {public:    string frequencySort(string s) {        unordered_map<char,int> freq;vector<string> bucket(s.size() + 1, "");string res;for (char c : s){freq[c]++;}for (auto it : freq){int n = it.second;char c = it.first;bucket[n].append(n, c);}for (int i = s.size(); i > 0; i--){if (!bucket[i].empty())res += bucket[i];}return res;    }};