leetcode 451. Sort Characters By Frequency

 

Medium

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.

class Solution {
public:
    string frequencySort(string s) {
        int len = s.size();
        // 建立一个hash表
        unordered_map<char, int> hash;
        // 建立频率和字符的对应关系
        vector<string> vec(len+1, "");
        
        for(auto ch: s) hash[ch]++;
        
        for(auto val: hash) vec[val.second].append(val.second, val.first);
        
        string ans;
        for(int i = len; i > 0; i--)
            ans += vec[i];
        return ans;

    }
};