leetcode 之 Maximum Depth of Binary Tree

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问题：

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.

方法一. 广度优先遍历，利用队列，在每一层的结尾插入一个标志表示该层结束。

#include <queue>#include <limits>using namespace std;/*** 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) {        if(root == NULL)            return 0;        queue<TreeNode *> tree_queue;        tree_queue.push(root);        TreeNode levelNode(std::numeric_limits<int>::max());        int max_depth = 0;        while( !tree_queue.empty() ){            tree_queue.push(&levelNode);            TreeNode * top = tree_queue.front();            tree_queue.pop();            //add all the children            while(top->val != std::numeric_limits<int>::max())            {                if(top->left){                    tree_queue.push(top->left);                }                if(top->right){                    tree_queue.push(top->right);                }                top = tree_queue.front();                tree_queue.pop();            }            //add depth when traverse one level            max_depth ++;        }        return max_depth;    }};

方法二 . 深度优先遍历，利用栈进行回溯。在每一层的结尾插入一个标志表示该层结束。在回溯时，一旦遇到标志位，减少当前的深度。

#include <stack>#include <limits>/*** 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) {        if(root == NULL)            return 0;        int max_depth = 0;        std::stack<TreeNode *> tree_stack;        tree_stack.push(root);        TreeNode *current = NULL;        TreeNode level(std::numeric_limits<int>::max());        int current_depth = 0;        while(!tree_stack.empty())        {            //pop top node.            current = tree_stack.top();            tree_stack.pop();            //visit level node.            if(current->val == std::numeric_limits<int>::max()){                current_depth --;            }            //insert new node.            else{                //insert a level flag.                tree_stack.push(&level);                current_depth ++;                if(max_depth < current_depth)                     max_depth = current_depth;                if(current->right)                    tree_stack.push(current->right);                if(current->left)                    tree_stack.push(current->left);            }        }        return max_depth;    }};

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