Add solution for "Reverse String" with unit tests.

src/main/java/com/leetcode/easy/ReverseString344.java

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+// Tags: String, Two Pointers
+package com.leetcode.easy;
+
+public class ReverseString344 {
+    /**
+     * Time Complexity: O(n) where n is the length of the array
+     * - We iterate through half of the array, swapping elements from both ends
+     * - Each element is visited exactly once
+     * <p>
+     * Space Complexity: O(1)
+     * - Only uses a constant amount of extra space (variables l, r, and temp)
+     * - The reversal is done in-place without allocating additional data structures
+     */
+    public void reverseString(char[] s) {
+        int l = 0;
+        int r = s.length - 1;
+        while (l < r) {
+            char temp = s[l];
+            s[l] = s[r];
+            s[r] = temp;
+            l++;
+            r--;
+        }
+    }
+}

src/test/java/com/leetcode/easy/ReverseString344Test.java

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+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+public class ReverseString344Test {
+    private ReverseString344 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new ReverseString344();
+    }
+
+    @Test
+    void testReverseString_Example1() {
+        char[] input = {'h', 'e', 'l', 'l', 'o'};
+        char[] expected = {'o', 'l', 'l', 'e', 'h'};
+        solution.reverseString(input);
+        assertArrayEquals(expected, input);
+    }
+
+    @Test
+    void testReverseString_Example2() {
+        char[] input = {'H', 'a', 'n', 'n', 'a', 'h'};
+        char[] expected = {'h', 'a', 'n', 'n', 'a', 'H'};
+        solution.reverseString(input);
+        assertArrayEquals(expected, input);
+    }
+}