Maximum Difference Between Node and Ancestor
<p>Given the <code>root</code> of a binary tree, find the maximum value <code>v</code> for which there exist <strong>different</strong> nodes <code>a</code> and <code>b</code> where <code>v = |a.val - b.val|</code> and <code>a</code> is an ancestor of <code>b</code>.</p>

<p>A node <code>a</code> is an ancestor of <code>b</code> if either: any child of <code>a</code> is equal to <code>b</code>&nbsp;or any child of <code>a</code> is an ancestor of <code>b</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1026.Maximum%20Difference%20Between%20Node%20and%20Ancestor/images/tmp-tree.jpg" style="width: 400px; height: 390px;" />
<pre>
<strong>Input:</strong> root = [8,3,10,1,6,null,14,null,null,4,7,13]
<strong>Output:</strong> 7
<strong>Explanation: </strong>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.</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1026.Maximum%20Difference%20Between%20Node%20and%20Ancestor/images/tmp-tree-1.jpg" style="width: 250px; height: 349px;" />
<pre>
<strong>Input:</strong> root = [1,null,2,null,0,3]
<strong>Output:</strong> 3
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[2, 5000]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
</ul>
Tree; Depth-First Search; Binary Tree
/**
 * 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 {
    private int ans;

    public int maxAncestorDiff(TreeNode root) {
        dfs(root, root.val, root.val);
        return ans;
    }

    private void dfs(TreeNode root, int mi, int mx) {
        if (root == null) {
            return;
        }
        int x = Math.max(Math.abs(mi - root.val), Math.abs(mx - root.val));
        ans = Math.max(ans, x);
        mi = Math.min(mi, root.val);
        mx = Math.max(mx, root.val);
        dfs(root.left, mi, mx);
        dfs(root.right, mi, mx);
    }
}