Subject requirements

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.

According to the number of occurrences of each letter, a new string is generated based on the number of occurrences. Pay attention to case sensitivity here.

Ideas and codes

Intuitively, if you can record the number of times each letter appears, then sort the letters according to the number of times the letters appear, and then form a new string. Here, the flow method is used for sorting, and the code is as follows:

    public String frequencySort(String s) {
        if(s==null || s.isEmpty() || s.length() <= 1) return s;
        Map<Character, StringBuilder> map = new HashMap<>();
        for(char c : s.toCharArray()) {
            map.put(c, map.getOrDefault(c, new StringBuilder()).append(c));
        }
        StringBuilder result = map
                .values()
                .stream()
                .sorted((sb1, sb2) -> {
                    return sb2.length() - sb1.length();
                })
                .reduce((sb1, sb2) -> sb1.append(sb2))
                .get();
        return result.toString();
    }

If it is not intuitive to sort, just find the most frequent letters from the map recording the number of letters each time, output them, and select the most frequent letters from the remaining letters again. Loop until all the letters are output. The code is as follows:

    public String frequencySort(String s) {
        char[] charArr = new char[128];
        
        
        for(char c :  s.toCharArray()) 
            charArr[c]++;
        
        StringBuilder sb = new StringBuilder();

        
        while(sb.length() < s.length()) {
                    char maxChar = 0;
            for(char charCur = 0; charCur < charArr.length; charCur++) {
                
                if(charArr[charCur] > charArr[maxChar]) {
                    maxChar = charCur;
                }
            }
            while(charArr[maxChar] > 0){
                sb.append(maxChar);
                charArr[maxChar]--;
            }
           
        }
        
        return sb.toString();
    }