101.Symmetric_Tree

# 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
class Solution:
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        if not root: return False
        def dfs(left, right):
            if left is None or right is None:
                return left is right # both as None
            return left.val == right.val and dfs(left.right, right.left) and \
            dfs(right.right, left.left)
        return dfs(root.left, root.right) if root.left is not root.right else True