Leetcode 451. Sort Characters By Frequency

https://leetcode.com/problems/sort-characters-by-frequency/

Problem Statement

Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string.

Return the sorted string. If there are multiple answers, return any of them.

Example

Input: s = "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.

Solution

    public String frequencySort(String s) {
        Map<Character, Integer> map = new HashMap<>();
        for (char ch : s.toCharArray()) {
            if (map.containsKey(ch)) {
                map.put(ch, map.get(ch) + 1);
            } else {
                map.put(ch, 1);
            }
        }
        List<Map.Entry<Character,Integer>> list = new LinkedList<Map.Entry<Character,Integer>>(map.entrySet());
        Collections.sort(list, (a, b) -> b.getValue() - a.getValue());
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<Character, Integer> entry : list) {
            for (int i = 0; i < entry.getValue(); i++) {
                sb.append(entry.getKey());
            }
        }
        return sb.toString();
    }