Untitled

from collections import deque

# 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:
        # use a queue, traverse the tree left to right and check every time it's traversed
        if not root:
            return True

        q = [] # q = []
        q.append(root.left) # [2]
        q.append(root.right) # [2, 2]

        # bfs = stack
        # dfs = queue

        # q = [2, 2], left = 2, right = 2
        while q:
            left = q.pop(0) # 2
            right = q.pop(0) # 2

            if not left and not right:
                continue

            if (not left and right) or (not right and left):
                return False

            if left.val != right.val: 
                return False

            # add outside mirror
            q.append(left.left)
            q.append(right.right)

            # and inside mirror
            q.append(left.right)
            q.append(right.left)

        return True