LeetCode 451. Sort Characters By Frequency

Description

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

Given a string s, sort it in decreasing order based on the frequency of characters, and return the sorted string.

Example 1:

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.

Example 2:

Input: s = "cccaaa"
Output: "aaaccc"
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: s = "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.

Constraints:

  • 1 <= s.length <= 5 * 105
  • s consists of English letters and digits.

Explanation

Count the occurrences of characters and base on the number of occurrences to build a new string.

Python Solution

class Solution:
    def frequencySort(self, s: str) -> str:
        counter = {}
        
        for c in s:
            counter[c] = counter.get(c, 0) + 1
            
        priority_queue = []
        
        for key, value in counter.items():        
            heapq.heappush(priority_queue, (-value, key))
        
        results = ""
        for k in range(len(priority_queue)):      
            pair = heapq.heappop(priority_queue)

            for i in range(-pair[0]):
                results += pair[1]

        
        return results
        
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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