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);
    }
}