104. Maximum Depth of Binary Tree

https://www.nowcoder.com/practice/435fb86331474282a3499955f0a41e8b?tpId=13&tqId=11191&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking
| 日期 | 是否一次通过 | comment |
|----|----|----|
|2019-01-26 13:20|Y||
|2019-01-27 13:20|N|对helper中的递归理解不够深刻|

题目：输入一棵二叉树，求该树的深度。从根结点到叶结点依次经过的结点（含根、叶结点）形成树的一条路径，最长路径的长度为树的深度。

1. 递归

public class Solution {
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
         
        TreeNode nodeD = new TreeNode(0);
        helper(nodeD, root);
         
        return nodeD.val + 1;
    }
     
    private int helper(TreeNode nodeD, TreeNode root) {
        if(root == null) {
            return 0;
        }
         
        int leftD = helper(nodeD, root.left);
        int rightD = helper(nodeD, root.right);
        nodeD.val = Math.max(Math.max(leftD, rightD), nodeD.val);
         
        return Math.max(leftD, rightD) + 1;
         
    }
}

2.递归

public class Solution {
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
         
        int leftDpt = TreeDepth(root.left);
        int rightDpt = TreeDepth(root.right);
         
        return Math.max(leftDpt, rightDpt) + 1;
    }
}