Add SortCharacterByFrequency implementation with unit tests

Author: lieutenant-Rohit

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

@@ -1,6 +1,8 @@
 // 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
@@ -16,7 +18,7 @@
  Space Complexity: O(n)
 */
 
-class Solution {
+class SortCharacterByFrequency {
     public String frequencySort(String s) {
 
         // Step 1: Count frequency of each character

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

@@ -38,4 +38,12 @@ void testMixedCharacters() {
         // 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"));
+    }
 }