LeetCode 1026. Maximum Difference Between Node and Ancestor(Medium)

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本文探讨了在二叉树中寻找最大祖先差值的问题，即找到两个节点A和B，使得A是B的祖先且两者值之差最大。通过递归遍历树并记录每个节点的祖先的最大和最小值，最终找到最大的差值。

Description:
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:
在这里插入图片描述

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.

Note:

The number of nodes in the tree is between 2 and 5000.
Each node will have value between 0 and 100000.

Analysis:

Define an object variable maxDiff to denote the solution to the problem.
Define a function findMaxDiff(TreeNode node, int max, int min) to find the max value of maxDiff as well as do the pre-order traversal of this tree. Here node represents the current node, maxand min represent the max value and min value of the ancestors of node respecticely.
In each execution of findMaxDiff function:
① Update the max, min, maxDiff.
② Recall next findMaxDiff function.

Code:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int maxDiff;
    public int maxAncestorDiff(TreeNode root) {
        if(root == null) {
            return maxDiff;
        }else{
            findMaxDiff(root, root.val, root.val);
            return maxDiff;
        }
    }
    
    public void findMaxDiff(TreeNode node, int max, int min) {
        if(node == null) {
            return;
        }else{
            if(node.val > max) {
                max = node.val;
            }
            if(node.val < min) {
                min = node.val;
            }
            
            int diff = max - min;
            if(diff > maxDiff) {
                maxDiff = diff;
            }
            
            if(node.left != null) {
                findMaxDiff(node.left, max, min);
            }
            
            if(node.right != null) {
                findMaxDiff(node.right, max, min);
            }
        }
    }
}