1372. Longest ZigZag Path in a Binary Tree

Python

DFS

class Solution:
    def longestZigZag(self, root: Optional[TreeNode]) -> int:

        def dfs(node, pre_direction, total):
            if not node:
                return total
            
            # If pre_direction is '', both sub-dfs will use 1 as 3rd arg
            return max(
                dfs(node.left, 'left', total+1 if pre_direction == 'right' else 1),
                dfs(node.right, 'right', total+1 if pre_direction == 'left' else 1)
            )
        return dfs(root, '', 0) - 1