My simple BFS solution


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    class Solution(object):

    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        queue=[root];stack=[]
        while queue and root:
            queue=[leaf for node in queue if node for leaf in (node.left,node.right)]
            stack=[node.val if node else 'null' for node in queue]
            if stack[::-1]!=stack: return False
        return True

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