[Leetcode] 451. Sort Characters By Frequency 解题报告

题目：

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.

思路：

这道题目如果采用基于比较的排序，则思路也很简单，不过时间复杂度最少需要O(nlogn)。但是如果采用计数排序，则可以将时间复杂度降低到O(n)。所以这道题目的考点我感觉其实是计数排序，请参考下面的代码。

代码：

class Solution {public:    string frequencySort(string s) {        int len = s.size();        unordered_map<char, int> hash;        for(auto ch : s) {            ++hash[ch];        }        vector<string> vec(len + 1, "");        for(auto val : hash) {            vec[val.second].append(val.second, val.first);        }        string ret;        for(int i = len; i >= 0; --i) {            ret += vec[i];        }        return ret;    }};