104 Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.
The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
#include <iostream>
#include <algorithm>
using namespace std;
struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* Recursive call stack will build towards the maximum depth of the left subtree,
* and then the right subtree. Leaving the call stack will return the right
* subtree tree and then left bottom up.
int maxDepth(TreeNode* root) {
if (root == NULL) return 0;
return max(maxDepth(root->left), maxDepth(root->right)) + 1;