给定一个二叉树，找出其最大深度。
二叉树的深度为根节点到最远叶子节点的距离。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: An integer
     */
    int maxDepth(TreeNode *root) {
        // write your code here
        if(root!=NULL){
            int left=maxDepth(root->left);
            int right=maxDepth(root->right);
            return left>right?left+1:right+1;
        }
        else{
            return 0;
        }
    }
};