【leetcode每日一题】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.

即是求一课二叉树的深度。

解析：

方法一：用递归方法做，代码如下：

/**
 * 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 l=maxDepth(root->left);
        int r=maxDepth(root->right);
        return l>r?l+1:r+1;
    }
};

方法二：非遍历方法，用队列来做。类似于广度遍历的方法，把二叉树节点按层压入队列中。没遍历完一层，则树的高度就加一。代码如下：

/**
 * 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;
	    }
	    queue<TreeNode *> nodes;
	    int height=0;
	    nodes.push(root);
	    TreeNode *temp;
	    int size=1;
	    while(!nodes.empty())
	    {
		    size--;
		    temp=nodes.front();
		    nodes.pop();
		    if(temp->left!=NULL)
		    {
			    nodes.push(temp->left);
		    }
		    if(temp->right!=NULL)
		    {
			    nodes.push(temp->right);
		    }
		    if(size==0)
		    {
			    size=nodes.size();
			    height++;
		    }
	    }
	    return height;
    }
};