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.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
return its depth = 3.
给一个二叉树，将他的最大深求出来并且进行输出
1.采用递归的思想，停止递归的条件是根节点没有左子树并且没有右子树，就返回0，
2.递归做什么事情，递归左子树，递归右子树，找到两个子树深度最大值，再进行加3.返回的就是两个子树深度最大值加1
package com.cxy.recursive;
import com.cxy.linkedtable.TreeNode;
import java.util.List;
/**
* 二叉树的深度优先搜索递归实现
*/
public class BinaryTreeDepth {
public int maxDepth(TreeNode root) {
if(root == null){return 0;}
int leftDeep = maxDepth(root.left);
int rightDeep = maxDepth(root.right);
return Math.max(leftDeep, rightDeep)+1;
}
public static void main(String[] args){
TreeNode left = new TreeNode(1, null, null);
TreeNode right = new TreeNode(2, null, null);
TreeNode root = new TreeNode(0, left, right);
int depth = new BinaryTreeDepth().maxDepth(root);
System.out.println(depth);
}
}
