LeetCode 1026. Maximum Difference Between Node and Ancestor

原题链接在这里：https://leetcode.com/problems/maximum-difference-between-node-and-ancestor/

题目：

Given the root of a binary tree, find the maximum value V for which there exists different nodes A and B where V = |A.val - B.val| and A is an ancestor of B.

(A node A is an ancestor of B if either: any child of A is equal to B, or any child of A is an ancestor of B.)

Example 1:

Input: [8,3,10,1,6,null,14,null,null,4,7,13]
Output: 7
Explanation:
We have various ancestor-node differences, some of which are given below :
|8 - 3| = 5
|3 - 7| = 4
|8 - 1| = 7
|10 - 13| = 3
Among all possible differences, the maximum value of 7 is obtained by |8 - 1| = 7.

题解：

Perform DFS top down. On each level, Calculate maxmimum difference and update minimum and maximum value.

Time Complexity: O(n).

Space: O(h).

AC Java:

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 class Solution {
     int res = 0;

     public int maxAncestorDiff(TreeNode root) {
         if(root == null){
             return res;
         }

         dfs(root, root.val, root.val);
         return res;
     }

     private void dfs(TreeNode root, int min, int max){
         if(root == null){
             return;
         }

         res = Math.max(res, Math.max(Math.abs(root.val - min), Math.abs(max-root.val)));
         min = Math.min(min, root.val);
         max = Math.max(max, root.val);
         dfs(root.left, min, max);
         dfs(root.right, min, max);
     }
 }