Add solution for the "Lowest Common Ancestor of a Binary Search Tree" problem with unit tests.

src/main/java/com/leetcode/easy/LowestCommonAncestorOfABinarySearchTree235.java

@@ -0,0 +1,26 @@
+// Tags: Tree, DFS, Binary Search Tree
+package com.leetcode.easy;
+
+import com.leetcode.datastructure.TreeNode;
+
+public class LowestCommonAncestorOfABinarySearchTree235 {
+    /*
+        Time complexity: O(N),
+        where N is the number of nodes in the BST.
+        In the worst case we might be visiting all the nodes of the BST.
+
+        Space complexity: O(N).
+        This is because the maximum amount of space
+        utilized by the recursion stack would be N
+        since the height of a skewed BST could be N.
+     */
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+        if (p.val > root.val && q.val > root.val) {
+            return this.lowestCommonAncestor(root.right, p, q);
+        }
+        if (p.val < root.val && q.val < root.val) {
+            return this.lowestCommonAncestor(root.left, p, q);
+        }
+        return root;
+    }
+}

src/test/java/com/leetcode/easy/LowestCommonAncestorOfABinarySearchTree235Test.java

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+package com.leetcode.easy;
+
+import com.leetcode.datastructure.TreeNode;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class LowestCommonAncestorOfABinarySearchTree235Test {
+    private LowestCommonAncestorOfABinarySearchTree235 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new LowestCommonAncestorOfABinarySearchTree235();
+    }
+
+    @Test
+    void testLowestCommonAncestor_Example1() {
+        TreeNode root = new TreeNode(6,
+                new TreeNode(2, new TreeNode(0), new TreeNode(4, new TreeNode(3), new TreeNode(5))),
+                new TreeNode(8, new TreeNode(7), new TreeNode(9)));
+        TreeNode p = root.left; // Node with value 2
+        TreeNode q = root.right; // Node with value 8
+        TreeNode result = solution.lowestCommonAncestor(root, p, q);
+        assertEquals(root, result);
+    }
+
+    @Test
+    void testLowestCommonAncestor_Example2() {
+        TreeNode root = new TreeNode(6,
+                new TreeNode(2, new TreeNode(0), new TreeNode(4, new TreeNode(3), new TreeNode(5))),
+                new TreeNode(8, new TreeNode(7), new TreeNode(9)));
+        TreeNode p = root.left; // Node with value 2
+        TreeNode q = root.left.right; // Node with value 4
+        TreeNode result = solution.lowestCommonAncestor(root, p, q);
+        assertEquals(p, result);
+    }
+
+    @Test
+    void testLowestCommonAncestor_Example3() {
+        TreeNode root = new TreeNode(2, new TreeNode(1), null);
+        TreeNode p = root; // Node with value 2
+        TreeNode q = root.left; // Node with value 1
+        TreeNode result = solution.lowestCommonAncestor(root, p, q);
+        assertEquals(root, result);
+    }
+}