451. Sort Characters By Frequency**

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.

    public String frequencySort(String s) {
        if(s.length()<3) return s;
        int max = 0;
        int[] map = new int[256];
        for(char ch: s.toCharArray()){
            map[ch]++;
            max = Math.max(max,map[ch]);
        }
        String[] buckets = new String[max+1];
        for(int i=0;i<256;i++){
            String str = buckets[map[i]];
            if(map[i]>0)
            buckets[map[i]]=(str==null)?"" + (char)i:(str + (char) i);
        }
        StringBuilder strb = new StringBuilder();
        for(int i=max;i>=0;i--){
            if(buckets[i]!=null) 
                for(char ch:buckets[i].toCharArray())
                    for (int j=0;j<i;j++)
                        strb.append(ch);
        }
        return strb.toString();
    }

Method2：

    public String frequencySort(String s) {
        if(s.length()<3) return s;
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        char [] array = s.toCharArray();
        int max = 0;
        for(int i=0;i<array.length;i++){
            if(!map.containsKey(array[i])) map.put(array[i], 0);
            map.put(array[i], map.get(array[i])+1);
            max = Math.max(max,map.get(array[i]));
        }
        List<Character>[] arrayList = new List[max+1];
        for(Character c: map.keySet()){
            int count = map.get(c);
            if(arrayList[count]==null) arrayList[count] = new ArrayList();
            arrayList[count].add(c);
        }
        StringBuilder result = new StringBuilder();
        for(int i = max;i>0;i--){
            List<Character> list = arrayList[i];
            if(list!=null){
                for(Character c:list){
                    for(int j=i;j>0;j--){
                        result.append(c);
                    }
                }
            }
        }
        return result.toString();
    }

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