题目：

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) {
            return 0;
        }
        int left = maxDepth(root->left);
        int right = maxDepth(root->right);
        return (left > right) ? left + 1 : right + 1;
    }
};

3.二叉树的最大深度