104. Maximum Depth of Binary Tree

1. 问题描述

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.
Tags: Tree Depth-first Search
Similar Problems: (E) Balanced Binary Tree (E) Minimum Depth of Binary Tree
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/

2. 解题思路

• 可以利用递归方式实现，但在LeetCode上运行超时，考虑使用循环方式实现
• 利用队列，逐层计算：

3. 代码

class Solution {
public:
    //非递归方法(利用队列实现)
    int maxDepth(TreeNode* root) 
    {
        if (NULL == root)
        {
            return 0;
        }
        std::queue<TreeNode *> tQue;
        tQue.push(root);
        int ndepth = 0;
        while (!tQue.empty())
        {
            ndepth++;
            int nSize = tQue.size();            
            for (int i=0; i<nSize; i++)
            {
                TreeNode *pCur = tQue.front();
                tQue.pop();
                if (NULL != pCur->left)
                {
                    tQue.push(pCur->left);
                }
                if (NULL != pCur->right)
                {
                    tQue.push(pCur->right);
                }
            }
        }
        return ndepth;
    }
    //递归实现（运行时间超时）
    int maxDepth_1(TreeNode* root) 
    {
        if (NULL == root)
        {
            return 0;
        }
        return 1+(maxDepth(root->left)>maxDepth(root->right) ? maxDepth(root->left):maxDepth(root->right));
    }
};

4. 反思

•  思路类似树的广度优先遍历