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# 題目敘述

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

TreeNode 的 class 內容

TreeNode
// 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;
    }
}
# 解題思路
# Solution
import java.util.ArrayList;
import java.util.Collections;
class Solution {
    public int maxAncestorDiff(TreeNode root) {
        int Max = 0;
        ArrayList<TreeNode> tree = new ArrayList<>();
        toNode(root, tree);
        for (TreeNode t : tree) {
            ArrayList<Integer> arr = new ArrayList<>();
            int min = 0;
            int max = 0;
            getAllValue(t, arr);
            if ((arr.size() - 1) != 0) {
                Collections.sort(arr);
                min = arr.get(0);
                max = arr.get(arr.size() - 1);
                if (Max < Math.abs(t.val - min)) {
                    Max = Math.abs(t.val - min);
                }
                if (Max < Math.abs(t.val - max)) {
                    Max = Math.abs(t.val - max);
                }
            }
        }
        return Max;
    }
    public void toNode(TreeNode root, ArrayList<TreeNode> tree) {
        if (root != null) {
            tree.add(root);
        }
        if (root.left != null) {
            toNode(root.left, tree);
        }
        if (root.right != null) {
            toNode(root.right, tree);
        }
    }
    public void getAllValue(TreeNode root, ArrayList<Integer> arr) {
        if (root.left != null) {
            getAllValue(root.left, arr);
        }
        if (root.right != null) {
            getAllValue(root.right, arr);
        }
        if (root != null) {
            arr.add(root.val);
        }
    }
}




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