Longest zig-zag path in a binary tree

'''
Logic:
->traversed the tree.
->leaf node alone can't contribute in zig zag maximum path so it will return 0.
->zig-zag path can started from any node and in any direction,so answer
  get's updted at every node.
->Assume that parent of current node is the starting node of zig zag maximum path,
  then reversed direction of child of current node can only contribute in
  zig zag path.so we only returned that path zig zag's.
'''
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

def fun(root,dir,ans):
    if root==None:
        return 0
    print(root.val, dir,ans)
    a=fun(root.left,'L',ans)
    b=fun(root.right,'R',ans)
    ans[0]=max(ans[0],max(a+1,b+1))
    if dir=='L':
        return b+1
    else:
        return a+1

class Solution:
    def longestZigZag(self, root: Optional[TreeNode]) -> int:
        ans=[0]
        a=fun(root.left,'L',ans)
        b=fun(root.right,'R',ans)
        return max(max(a,b)+1,ans[0])-1 #reduced by 1 because edge will one less than the nodes.