451. Sort Characters By Frequency

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.

看到这个题确实不会做，看了一下答案。确实思考了很久才看懂答案，因为很多申明变量的时候还用的不熟练，Map-HashMap; List<> = new ArrayList[]; StringBuilder 等等。

首先是建立一个HashMap做Char 与出现次数的对应num；然后用一个桶Bucket的表List来装出现多少次的char。最后遍历这个桶，从前向后遍历然后用StringBuilder或者StringBuffer来储存，最后转换成String输出。学了python之后觉得这样好麻烦。