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.

 

此题类似二叉树最小深度。

/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

import java.util.*;

public class Solution {

    public int maxDepth(TreeNode root) {

        if(root==null)

            return 0;

        if(root.left==null&&root.right==null)

            return 1;

方法一：递归

    /*    if(root.left==null)

            return maxDepth(root.right)+1;

        if(root.right==null)

            return maxDepth(root.left)+1;

        int left=maxDepth(root.left);

        int right=maxDepth(root.right);

       

        return (left>right)?(left+1):(right+1);

       

        */

        //方法二：层序遍历结束，每一层level加一

        Queue<TreeNode> q=new LinkedList<>();

        q.add(root);

        int level=0;

        while(!q.isEmpty()){

            int size=q.size();

            level++;

            for(int i=0;i<size;i++){

                TreeNode node=q.poll();

                if(node.left!=null)

                    q.add(node.left);

                if(node.right!=null)

                    q.add(node.right);

            }

        }

       

        return level;

    }

}