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.

class Solution {
    public String frequencySort(String s) {
        HashMap<Character, Integer> map = new HashMap<>();
        int max = 0;
        for (char c : s.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
            max = Math.max(max, map.get(c));
        }
        ArrayList<Character>[] list = new ArrayList[max + 1];
        for (int i = 0; i <= max; i++)
            list[i] = new ArrayList<>();
        for (char c : map.keySet())
            list[map.get(c)].add(c);
        StringBuilder str = new StringBuilder();
        for (int i = max; i > 0; i--) {
            if (list[i].size() > 0)
                for (char c : list[i]) {
                    int count = i;
                    while (count-- > 0)
                        str.append(c);
                }
        }
        return str.toString();
    }
}