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.

思路：

递归求解二叉树的深度，左右子树较深的+1（。解释主要原因：存在根节点）

实现代码：

/**

 * 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==0)

        {

            return 0;

        }

        int left=maxDepth(root->left);

        int right=maxDepth(root->right);

        if(left>right)

        {

            return left+1;

        }

        else

        {

            return right+1;

        }

    }

};