1372. Longest ZigZag Path in a Binary Tree
给你一个二元树，找出这个二元树里的Z字型路径最大长度。
Example :
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).
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).
思路：
1.用dfs遍历树，把上一层的最大Z字型长度按照方向交错传给下一层，并在途中不断更
新 ans。
Java Code：