Add solution for the "Merge Sorted Array" problem with unit tests.

src/main/java/com/leetcode/easy/MergeSortedArray88.java

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+// Tags: Array, Two Pointers
+package com.leetcode.easy;
+
+public class MergeSortedArray88 {
+    /*
+        Time complexity: O(m + n)
+        Breakdown:
+        -   First while loop: Compares elements from both arrays and places them in the correct position
+            -   In each iteration, either p1 or p2 decrements by 1
+            -   The loop continues until one of the pointers reaches -1
+            -   Maximum iterations: min(m, n)
+        -   Second while loop: Copies remaining elements from nums2 (if any)
+            -   Runs at most n iterations (when all elements from nums2 need to be copied)
+        -   Total operations: Each element from both arrays is processed exactly once
+        -   Overall: O(m + n)
+        Where m is the number of valid elements in nums1 and n is the number of elements in nums2.
+
+        Space complexity: O(1)
+        Breakdown:
+        -   In-place modification: The algorithm modifies nums1 directly without using additional arrays
+        -   Pointer variables: p1, p2, and index use constant space O(1)
+        -   No extra data structures: No additional arrays, lists, or collections are created
+        -   Overall: O(1) - constant space complexity
+     */
+    public void merge(int[] nums1, int m, int[] nums2, int n) {
+        int p1 = m - 1;
+        int p2 = n - 1;
+        int index = m + n - 1;
+        while (p1 >= 0 && p2 >= 0) {
+            if (nums1[p1] >= nums2[p2]) {
+                nums1[index] = nums1[p1];
+                p1--;
+            } else {
+                nums1[index] = nums2[p2];
+                p2--;
+            }
+            index--;
+        }
+        while (p2 >= 0) {
+            nums1[index] = nums2[p2];
+            p2--;
+            index--;
+        }
+    }
+}

src/test/java/com/leetcode/easy/MergeSortedArray88Test.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 MergeSortedArray88Test {
+    private MergeSortedArray88 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new MergeSortedArray88();
+    }
+
+    @Test
+    void testMerge_Example1() {
+        int[] nums1 = {1, 2, 3, 0, 0, 0};
+        int m = 3;
+        int[] nums2 = {2, 5, 6};
+        int n = 3;
+
+        solution.merge(nums1, m, nums2, n);
+
+        int[] expected = {1, 2, 2, 3, 5, 6};
+        assertArrayEquals(expected, nums1);
+    }
+
+    @Test
+    void testMerge_Example2() {
+        int[] nums1 = {1};
+        int m = 1;
+        int[] nums2 = {};
+        int n = 0;
+
+        solution.merge(nums1, m, nums2, n);
+
+        int[] expected = {1};
+        assertArrayEquals(expected, nums1);
+    }
+
+    @Test
+    void testMerge_Example3() {
+        int[] nums1 = {0};
+        int m = 0;
+        int[] nums2 = {1};
+        int n = 1;
+
+        solution.merge(nums1, m, nums2, n);
+
+        int[] expected = {1};
+        assertArrayEquals(expected, nums1);
+    }
+
+    @Test
+    void testMerge_Example4() {
+        int[] nums1 = {4, 5, 6, 0, 0, 0};
+        int m = 3;
+        int[] nums2 = {1, 2, 3};
+        int n = 3;
+
+        solution.merge(nums1, m, nums2, n);
+
+        int[] expected = {1, 2, 3, 4, 5, 6};
+        assertArrayEquals(expected, nums1);
+    }
+
+    @Test
+    void testMerge_Example5() {
+        int[] nums1 = {1, 2, 3, 0, 0, 0};
+        int m = 3;
+        int[] nums2 = {4, 5, 6};
+        int n = 3;
+
+        solution.merge(nums1, m, nums2, n);
+
+        int[] expected = {1, 2, 3, 4, 5, 6};
+        assertArrayEquals(expected, nums1);
+    }
+}