原題鏈接：Symmetric Tree

class Solution:
    # @param {TreeNode} root
    # @return {boolean}
    def isSymmetric(self, root):
        if root is None:
            return True
        return self.isSy(root.left, root.right)
    def isSy(self, root1, root2):
        if root1 is None and root2 is None:
            return True
        elif root1 is None or root2 is None:
            return False
        else:
            if root1.val == root2.val:
                return self.isSy(root1.left, root2.right) and self.isSy(root1.right, root2.left)
            return False