LeetCode 451: Sort Characters By Frequency Solution

Master LeetCode problem 451 (Sort Characters By Frequency), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

451. Sort Characters By Frequency

Medium

Hash Table String Sorting Heap (Priority Queue)Bucket Sort Counting

Problem Explanation

Explanation

To solve this problem, we can follow these steps:

Count the frequency of each character in the input string.
Sort the characters based on their frequencies in descending order.
Reconstruct the sorted string by appending the characters based on their frequencies.

Time complexity: O(n log n)
Space complexity: O(n)

Solution Code

import java.util.*;

class Solution {
    public String frequencySort(String s) {
        Map<Character, Integer> charFreq = new HashMap<>();
        for (char c : s.toCharArray()) {
            charFreq.put(c, charFreq.getOrDefault(c, 0) + 1);
        }
        
        List<Character> characters = new ArrayList<>(charFreq.keySet());
        Collections.sort(characters, (a, b) -> charFreq.get(b) - charFreq.get(a));
        
        StringBuilder sb = new StringBuilder();
        for (char c : characters) {
            int freq = charFreq.get(c);
            for (int i = 0; i < freq; i++) {
                sb.append(c);
            }
        }
        
        return sb.toString();
    }
}

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