[LeetCode] 1026. Maximum Difference Between Node and Ancestor

Given the root of a binary tree, find the maximum value V for which there exist 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: root = [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.

Example 2:

Input: root = [1,null,2,null,0,3]
Output: 3

Constraints:

• The number of nodes in the tree is in the range [2, 5000].
• 0 <= Node.val <= 105

节点与其祖先之间的最大差值。

给定二叉树的根节点 root，找出存在于不同节点 A 和 B 之间的最大值 V，其中 V = |A.val - B.val|，且 A 是 B 的祖先。

（如果 A 的任何子节点之一为 B，或者 A 的任何子节点是 B 的祖先，那么我们认为 A 是 B 的祖先）

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/maximum-difference-between-node-and-ancestor
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思路是DFS前序遍历。题目让你找的是二叉树里面两个不同节点之间的最大差值，这两个节点要在同一条分支上。那么我们可以用DFS前序遍历，在遍历的过程中，记录一下当前路径上节点的最大值和最小值。当我们到达叶子节点的时候，则可以计算一下这个差值是多少。

时间O(n)

空间O(n)

Java实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int maxAncestorDiff(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return helper(root, root.val, root.val);
    }

    private int helper(TreeNode root, int max, int min) {
        // base case
        if (root == null) {
            return max - min;
        }
        max = Math.max(max, root.val);
        min = Math.min(min, root.val);
        int left = helper(root.left, max, min);
        int right = helper(root.right, max, min);
        return Math.max(left, right);
    }
}

LeetCode 题目总结