Add max depth binary tree implementation

data_structures/max_depth_binary_tree.cpp

@@ -0,0 +1,47 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+struct TreeNode {
+    int val;
+    TreeNode* left;
+    TreeNode* right;
+    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+};
+
+class Solution {
+public:
+    int maxDepth(TreeNode* root) {
+        if (!root) return 0;
+        return 1 + max(maxDepth(root->left), maxDepth(root->right));
+    }
+};
+
+TreeNode* newNode(int val) {
+    return new TreeNode(val);
+}
+
+int main() {
+    Solution sol;
+
+    //     3
+    //    / \
+    //   9  20
+    //      / \
+    //     15   7
+    TreeNode* root1 = newNode(3);
+    root1->left = newNode(9);
+    root1->right = newNode(20);
+    root1->right->left = newNode(15);
+    root1->right->right = newNode(7);
+    cout << "Test 1: " << sol.maxDepth(root1) << " (expected 3)" << endl;
+
+    // [1, null, 2]
+    TreeNode* root2 = newNode(1);
+    root2->right = newNode(2);
+    cout << "Test 2: " << sol.maxDepth(root2) << " (expected 2)" << endl;
+
+    // empty tree
+    cout << "Test 3: " << sol.maxDepth(nullptr) << " (expected 0)" << endl;
+
+    return 0;
+}