LeetCode题目笔记(二) -- Maximum Depth of Binary Tree

2. Problem: Maximum Depth of Binary Tree

Problem Description: 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.

Step I: Ask Questions

Complete binary tree? how is the binary tree implemented? What is depth?

Step II: Describe approaches to the problem-->Algorithm-->Algorithm Analysis. 

与上一题相似，我们也有recursive和iterative的解法。

http://www.geeksforgeeks.org/write-a-c-program-to-find-the-maximum-depth-or-height-of-a-tree/ 

Iterative的解法还是用BFS。这一次找在queue里面的最后一个元素的depth值。两种方法的代码如下：

class Solution {public:    int maxDepth(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if( root == NULL ) return 0;        if( root->left == NULL && root->right == NULL ) return 1;        else            return 1 + max( maxDepth( root->left ), maxDepth( root->right ) );    }};

class Solution {public:struct Node{    TreeNode *root;    int depth;    Node(TreeNode* n, int d):depth(d){        root = n;    }};    int maxDepth(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if( root==NULL ) return 0;                queue<Node> q;        q.push(Node( root, 1));                int maxDepth = 1;        while( !q.empty() )        {            Node u = q.front();            q.pop();                        if( u.root->left )                q.push( Node(u.root->left, u.depth+1));            if( u.root->right )                q.push( Node(u.root->right, u.depth+1));            if( u.root->left==NULL && u.root->right == NULL)            {                if( maxDepth < u.depth )                    maxDepth = u.depth;            }                    }                return maxDepth;    }};