Question：

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.

找二叉树的最大深度

Algorithm：

递归，两种写法

Accepted Code：

写法1、

/**
 * 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) {
        int deep=0;
        if(root!=NULL)
        {
            int deep_left = maxDepth(root->left);
            int deep_right = maxDepth(root->right);
            deep = (deep_left > deep_right)?deep_left+1:deep_right+1;
        }
        return deep;
    }
};

写法2、

/**
 * 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) {
        int res=0;
        helper(root,0,&res);
        return res;
    }
    void helper(TreeNode* root,int index,int *max)
    {
        if(root==NULL)
        {
            if(*max<index)
                *max=index;
            return;
        }
        else
        {
            helper(root->left,index+1,max);
            helper(root->right,index+1,max);
        }
    }
};