# 1372. Longest ZigZag Path in a Binary Tree

https://leetcode.com/problems/longest-zigzag-path-in-a-binary-tree

## Description

You are given the `root` of a binary tree.

A ZigZag path for a binary tree is defined as follow:

* Choose **any** node in the binary tree and a direction (right or left).
* If the current direction is right, move to the right child of the current node; otherwise, move to the left child.
* Change the direction from right to left or from left to right.
* Repeat the second and third steps until you can't move in the tree.

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

Return _the longest **ZigZag** path contained in that tree_.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/01/22/sample_1_1702.png)

```
**Input:** root = [1,null,1,1,1,null,null,1,1,null,1,null,null,null,1,null,1]
**Output:** 3
**Explanation:** Longest ZigZag path in blue nodes (right -> left -> right).
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/01/22/sample_2_1702.png)

```
**Input:** root = [1,1,1,null,1,null,null,1,1,null,1]
**Output:** 4
**Explanation:** Longest ZigZag path in blue nodes (left -> right -> left -> right).
```

**Example 3:**

```
**Input:** root = [1]
**Output:** 0
```

**Constraints:**

* The number of nodes in the tree is in the range `[1, 5 * 104]`.
* `1 <= Node.val <= 100`

## ac

```java
```
