Leetcode_451 Sort Characters By Frequency

原题地址：https://leetcode.com/problems/sort-characters-by-frequency/?tab=Description

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.

        看到这道题，第一反应用map来做。先用map来统计一下，再输出就行了。

        后来看到一种相对更简单的方法，用C++里<algorithm>的sort函数简单用一个既定规则排一下序就行了。程序代码如下（匿名函数此时挺好用）：

ps：此题标注难度为medium，但是感觉其实应属于easy。