Added Test Cases for SortCharacterByFrequency

Author: lieutenant-Rohit

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

@@ -1,3 +1,4 @@
+// Reference: https://leetcode.com/problems/sort-characters-by-frequency/
 import java.util.*;
 
 // Problem: Sort Characters By Frequency

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

@@ -0,0 +1,40 @@
+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"));
+    }
+}