Leetcode: 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.

Bucket Sort + HashMap

public class Solution {
    public String frequencySort(String s) {
        Map<Character, Integer> map = new HashMap<>();
        for (char c : s.toCharArray()) {
            if (map.containsKey(c)) {
                map.put(c, map.get(c) + 1);
            } else {
                map.put(c, 1);
            }
        }
        List<Character> [] bucket = new List[s.length() + 1];
        for (char key : map.keySet()) {
            int frequency = map.get(key);
            if (bucket[frequency] == null) {
                bucket[frequency] = new ArrayList<>();
            }
            bucket[frequency].add(key);
        }
        StringBuilder sb = new StringBuilder();
        for (int pos = bucket.length - 1; pos >=0; pos--) {
            if (bucket[pos] != null) {
                for (char num : bucket[pos]) {
                    for (int i = 0; i < map.get(num); i++) {
                        sb.append(num);
                    }
                }
            }
        }
        return sb.toString();
    }
}

HashMap+ Heap+Wrapper Class

public class Solution {
    public String frequencySort(String s) {
        PriorityQueue<WrapperChar> maxheap = new PriorityQueue(1, new Comparator<WrapperChar>() {
            public int compare(WrapperChar w1, WrapperChar w2) {
                return w2.count - w1.count;
            }
        });
        HashMap<Character, WrapperChar> map = new HashMap<Character, WrapperChar>();
        for (int i=0; i<s.length(); i++) {
            char c = s.charAt(i);
            if (!map.containsKey(c)) map.put(c, new WrapperChar(c, 1));
            else {
                int newCount = map.get(c).count + 1;
                map.put(c, new WrapperChar(c, newCount));
            }
        }
        for (char c : map.keySet()) maxheap.offer(map.get(c));
        StringBuilder res = new StringBuilder();
        while (!maxheap.isEmpty()) {
            WrapperChar wChar = maxheap.poll();
            for (int i=0; i<wChar.count; i++) {
                res.append(wChar.c);
            }
        }
        return res.toString();
    }
    
    public class WrapperChar {
        char c;
        int count;
        public WrapperChar(char ch, int num) {
            this.c = ch;
            this.count = num;
        }
    }
}