104. Maximum Depth of Binary Tree

题目：

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：

本题思路有两种思路：1.递归法  2.层次法

代码：

递归法：

/** * 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==NULL)            return 0;        else            return max(maxDepth(root->left),maxDepth(root->right))+1;            }};

层次法：

class Solution {public:    int maxDepth(TreeNode* root) {    if(root == NULL)        return 0;    int res = 0;    queue<TreeNode *> q;    q.push(root);    while(!q.empty())    {        ++ res;        for(int i = 0, n = q.size(); i < n; ++ i)        {            TreeNode *p = q.front();            q.pop();            if(p -> left != NULL)                q.push(p -> left);            if(p -> right != NULL)                q.push(p -> right);        }    }    return res;        }};