Sort character by frequency

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

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+// Reference: https://leetcode.com/problems/sort-characters-by-frequency/
+package com.thealgorithms.strings;
+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 SortCharacterByFrequency {
+    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();
+    }
+}

src/test/java/com/thealgorithms/strings/SortCharacterByFrequencyTest.java

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+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class SortCharacterByFrequencyTest {
+
+    private final SortCharacterByFrequency solution = new SortCharacterByFrequency();
+
+    @Test
+    void testBasicCase() {
+        String result = solution.frequencySort("tree");
+
+        // Possible outputs: "eert" or "eetr"
+        assertTrue(result.equals("eert") || result.equals("eetr"));
+    }
+
+    @Test
+    void testSingleCharacter() {
+        assertEquals("a", solution.frequencySort("a"));
+    }
+
+    @Test
+    void testEmptyString() {
+        assertEquals("", solution.frequencySort(""));
+    }
+
+    @Test
+    void testAllSameCharacters() {
+        assertEquals("aaaa", solution.frequencySort("aaaa"));
+    }
+
+    @Test
+    void testMixedCharacters() {
+        String result = solution.frequencySort("cccaaa");
+
+        // Possible outputs: "cccaaa" or "aaaccc"
+        assertTrue(result.equals("cccaaa") || result.equals("aaaccc"));
+    }
+
+    @Test
+    void testWithNumbersAndLetters() {
+        String result = solution.frequencySort("Aabb");
+
+        // 'b' appears twice → should come first
+        assertTrue(result.startsWith("bb"));
+    }
+}