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;
}
}
}

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