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:

    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:

    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`