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

src/main/java/com/leetcode/medium/LowestCommonAncestorOfABinaryTree236.java

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+// Tags: Tree, DFS
+package com.leetcode.medium;
+
+import com.leetcode.datastructure.TreeNode;
+
+public class LowestCommonAncestorOfABinaryTree236 {
+    /*
+        Time complexity: O(N),
+        where N is the number of nodes in the binary tree.
+        In the worst case we might be visiting all the nodes of the binary tree.
+
+        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 binary tree could be N.
+     */
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+        if (root == p || root == q || root == null) {
+            return root;
+        }
+        TreeNode left = lowestCommonAncestor(root.left, p, q);
+        TreeNode right = lowestCommonAncestor(root.right, p, q);
+        if (left != null && right != null) {
+            return root;
+        }
+        if (left != null) {
+            return left;
+        }
+        return right;
+    }
+}

src/test/java/com/leetcode/medium/LowestCommonAncestorOfABinaryTree236Test.java

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+package com.leetcode.medium;
+
+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 LowestCommonAncestorOfABinaryTree236Test {
+    private LowestCommonAncestorOfABinaryTree236 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new LowestCommonAncestorOfABinaryTree236();
+    }
+
+    @Test
+    void testLowestCommonAncestor_Example1() {
+        TreeNode root = new TreeNode(3,
+                new TreeNode(5, new TreeNode(6), new TreeNode(2, new TreeNode(7), new TreeNode(4))),
+                new TreeNode(1, new TreeNode(0), new TreeNode(8)));
+        TreeNode p = root.left; // Node with value 5
+        TreeNode q = root.right; // Node with value 1
+        TreeNode result = solution.lowestCommonAncestor(root, p, q);
+        assertEquals(root, result);
+    }
+
+    @Test
+    void testLowestCommonAncestor_Example2() {
+        TreeNode root = new TreeNode(3,
+                new TreeNode(5, new TreeNode(6), new TreeNode(2, new TreeNode(7), new TreeNode(4))),
+                new TreeNode(1, new TreeNode(0), new TreeNode(8)));
+        TreeNode p = root.left; // Node with value 5
+        TreeNode q = root.left.right.right; // Node with value 4
+        TreeNode result = solution.lowestCommonAncestor(root, p, q);
+        assertEquals(p, result);
+    }
+
+    @Test
+    void testLowestCommonAncestor_Example3() {
+        TreeNode root = new TreeNode(1, new TreeNode(2), null);
+        TreeNode p = root; // Node with value 1
+        TreeNode q = root.left; // Node with value 2
+        TreeNode result = solution.lowestCommonAncestor(root, p, q);
+        assertEquals(p, result);
+    }
+}