Longest ZigZag Path in a Binary Tree
<p>You are given the <code>root</code> of a binary tree.</p>

<p>A ZigZag path for a binary tree is defined as follow:</p>

<ul>
	<li>Choose <strong>any </strong>node in the binary tree and a direction (right or left).</li>
	<li>If the current direction is right, move to the right child of the current node; otherwise, move to the left child.</li>
	<li>Change the direction from right to left or from left to right.</li>
	<li>Repeat the second and third steps until you can&#39;t move in the tree.</li>
</ul>

<p>Zigzag length is defined as the number of nodes visited - 1. (A single node has a length of 0).</p>

<p>Return <em>the longest <strong>ZigZag</strong> path contained in that tree</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1300-1399/1372.Longest%20ZigZag%20Path%20in%20a%20Binary%20Tree/images/sample_1_1702.png" style="width: 221px; height: 383px;" />
<pre>
<strong>Input:</strong> root = [1,null,1,1,1,null,null,1,1,null,1,null,null,null,1]
<strong>Output:</strong> 3
<strong>Explanation:</strong> Longest ZigZag path in blue nodes (right -&gt; left -&gt; right).
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1300-1399/1372.Longest%20ZigZag%20Path%20in%20a%20Binary%20Tree/images/sample_2_1702.png" style="width: 157px; height: 329px;" />
<pre>
<strong>Input:</strong> root = [1,1,1,null,1,null,null,1,1,null,1]
<strong>Output:</strong> 4
<strong>Explanation:</strong> Longest ZigZag path in blue nodes (left -&gt; right -&gt; left -&gt; right).
</pre>

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

<pre>
<strong>Input:</strong> root = [1]
<strong>Output:</strong> 0
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 5 * 10<sup>4</sup>]</code>.</li>
	<li><code>1 &lt;= Node.val &lt;= 100</code></li>
</ul>
Tree; Depth-First Search; Dynamic Programming; 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 longestZigZag(TreeNode root) {
        dfs(root, 0, 0);
        return ans;
    }

    private void dfs(TreeNode root, int l, int r) {
        if (root == null) {
            return;
        }
        ans = Math.max(ans, Math.max(l, r));
        dfs(root.left, r + 1, 0);
        dfs(root.right, 0, l + 1);
    }
}