# 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: TreeNode) -> bool:
        from collections import deque
        d = deque()
        d.append((root,root))
        while d:
            left,right = d.popleft()
            if not left and not right:
                continue
            elif not left or not right:
                return False
            elif left.val != right.val:
                return False
            else:
                d.append((left.left,right.right))
                d.append((left.right,right.left))
        return True