题目描述：

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.

代码实现：

/**
 * 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) {
        int Ldepth=0,Rdepth=0;
        if(root==NULL){
            return 0;
        }
        if(root->left==NULL && root->right==NULL){
            return 1;
        }
        Ldepth=maxDepth(root->left);
        Rdepth=maxDepth(root->right);
      
        return 1+(Ldepth>Rdepth?Ldepth:Rdepth);
    }
};