451. Sort Characters By Frequency

Posted 2021-02-06 mengchunchen

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.

按照字符出现次数对字符串排序

C++:

 1 class Solution {
 2 public:
 3     string frequencySort(string s) {
 4         unordered_map<char,int> cntMap ;
 5         for(char c : s){
 6             cntMap[c]++ ;
 7         }
 8         vector<vector<char>> buckets(s.size()+1) ;
 9         for(auto m : cntMap){
10             buckets[m.second].push_back(m.first) ;
11         }
12         string res = "" ;
13         for(int i = buckets.size()-1 ; i >= 0 ; i--){
14             for(char c : buckets[i]){
15                 for(int j = 0 ; j < i ; j++){
16                     res += c ;
17                 }
18             }
19         }
20         return res ;
21     }
22 };