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.

/**
 * Definition for a binary tree node.
 * 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 == NULL)
            return 0;  
        int res = 1;  
        int l = maxDepth(root->left);  
        int r = maxDepth(root->right);  
        return l > r? l + 1:r+ 1;  
    }
};