Diameter of N-Ary Tree
<p>Given a <code>root</code> of an <code>N-ary tree</code>, you need to compute the length of the diameter of the tree.</p>

<p>The diameter of an N-ary tree is the length of the <strong>longest</strong> path between any two nodes in the tree. This path may or may not pass through the root.</p>

<p>(<em>Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value.)</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<p><img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1500-1599/1522.Diameter%20of%20N-Ary%20Tree/images/sample_2_1897.png" style="width: 324px; height: 173px;" /></p>

<pre>
<strong>Input:</strong> root = [1,null,3,2,4,null,5,6]
<strong>Output:</strong> 3
<strong>Explanation: </strong>Diameter is shown in red color.</pre>

<p><strong class="example">Example 2:</strong></p>

<p><strong><img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1500-1599/1522.Diameter%20of%20N-Ary%20Tree/images/sample_1_1897.png" style="width: 253px; height: 246px;" /></strong></p>

<pre>
<strong>Input:</strong> root = [1,null,2,null,3,4,null,5,null,6]
<strong>Output:</strong> 4
</pre>

<p><strong class="example">Example 3:</strong></p>

<p><img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1500-1599/1522.Diameter%20of%20N-Ary%20Tree/images/sample_3_1897.png" style="width: 369px; height: 326px;" /></p>

<pre>
<strong>Input:</strong> root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
<strong>Output:</strong> 7
</pre>

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

<ul>
	<li>The depth of the n-ary tree is less than or equal to <code>1000</code>.</li>
	<li>The total number of nodes is between <code>[1, 10<sup>4</sup>]</code>.</li>
</ul>
Tree; Depth-First Search
/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;


    public Node() {
        children = new ArrayList<Node>();
    }

    public Node(int _val) {
        val = _val;
        children = new ArrayList<Node>();
    }

    public Node(int _val,ArrayList<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    private int ans;

    public int diameter(Node root) {
        ans = 0;
        dfs(root);
        return ans;
    }

    private int dfs(Node root) {
        if (root == null) {
            return 0;
        }
        int m1 = 0, m2 = 0;
        for (Node child : root.children) {
            int t = dfs(child);
            if (t > m1) {
                m2 = m1;
                m1 = t;
            } else if (t > m2) {
                m2 = t;
            }
        }
        ans = Math.max(ans, m1 + m2);
        return 1 + m1;
    }
}