MayChallenge (52) | Sort Character By Frequently

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.

If you use arr to save the map, a program will be allocate a memory consecutive, it is not good idea for space memory. So, we are use Heap to sort and Map to save the map, with PriorityQueue(Heap), we use a tiny Lamba in Java8 for quick compare, take a look my source below:

PritoryQueue implement by MaxHeap, in book of Princeton 

	class Solution {
	public String frequencySort(String s) {
	HashMap<Character,Integer> map = new HashMap<>();
	PriorityQueue<Character> pq = new PriorityQueue((a,b)->(map.get(b) - map.get(a)));
	for (char i:s.toCharArray()){
	map.put(i,map.getOrDefault(i,0)+1);
	}
	pq.addAll(map.keySet());
	StringBuilder res = new StringBuilder();
	while (!pq.isEmpty()){
	char c = pq.remove();
	for (int i=0; i<map.get(c); i++){
	res.append(c);
	}
	}
	return res.toString();
	}
	}

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