leetcode Maximum Depth of Binary Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode *root) 
    {
        if (!root) return 0;
        int l = maxDepth(root -> left);
        int r = maxDepth(root -> right);
        return max(l + 1, r + 1);
    }
};