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.
HashMap sort in Map.Entry<>
public class Solution { public String frequencySort(String s) { HashMap<Character, Integer> map = new HashMap<>(); for(int i = 0; i < s.length(); i ++){ map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1); } ArrayList<Map.Entry<Character, Integer>> set = new ArrayList<Map.Entry<Character, Integer>>(map.entrySet()); Collections.sort(set, new Comparator<Map.Entry<Character, Integer>>(){ public int compare(Map.Entry<Character, Integer> v1, Map.Entry<Character, Integer> v2){ return v2.getValue()-v1.getValue(); } }); StringBuilder sb = new StringBuilder(); for(Map.Entry<Character, Integer> k : set){ int v = k.getValue(); for(int i = 0 ; i < v ; i++){ sb.append(k.getKey().toString()); } } return sb.toString(); } }