Sort Character by Frequency

Author: lieutenant-Rohit

src/main/java/com/thealgorithms/strings/SortCharacterByFrequency.java

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+import java.util.*;
+
+// Problem: Sort Characters By Frequency
+// Pattern: HashMap + Priority Queue (Heap)
+// Difficulty: Medium
+
+/*
+ Approach:
+ 1. Count frequency of each character using a HashMap.
+ 2. Store characters in a max-heap (PriorityQueue) based on frequency.
+ 3. If frequencies are same, sort by character (lexicographically).
+ 4. Extract from heap and append characters 'frequency' times.
+
+ Time Complexity: O(n log n)
+ Space Complexity: O(n)
+*/
+
+class Solution {
+    public String frequencySort(String s) {
+
+        // Step 1: Count frequency of each character
+        Map<Character, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < s.length(); i++) {
+            char ch = s.charAt(i);
+
+            // Increment frequency using getOrDefault
+            map.put(ch, map.getOrDefault(ch, 0) + 1);
+        }
+
+        // Step 2: Create max-heap based on frequency
+        PriorityQueue<Map.Entry<Character, Integer>> pq =
+                new PriorityQueue<>((a, b) -> {
+
+                    // Higher frequency comes first
+                    int diff = b.getValue() - a.getValue();
+
+                    // If frequency same, sort by character
+                    if (diff == 0) return a.getKey() - b.getKey();
+
+                    return diff;
+                });
+
+        // Add all entries to heap
+        pq.addAll(map.entrySet());
+
+        // Step 3: Build result string
+        StringBuilder sb = new StringBuilder();
+
+        while (!pq.isEmpty()) {
+
+            Map.Entry<Character, Integer> entry = pq.poll();
+
+            char key = entry.getKey();   // character
+            int freq = entry.getValue(); // frequency
+
+            // Append character 'freq' times
+            for (int i = 0; i < freq; i++) {
+                sb.append(key);
+            }
+        }
+
+        // Step 4: Return final string
+        return sb.toString();
+    }
+}