class Solution:
def isSymmetric(self, root: TreeNode) -> bool:
if not root:
return True
return self.compare(root.left, root.right)

def compare(self, left, right):
    #首先排除空节点的情况
    if left == None and right != None: return False
    elif left != None and right == None: return False
    elif left == None and right == None: return True
    #排除了空节点，再排除数值不相同的情况
    elif left.val != right.val: return False
    
    #此时就是：左右节点都不为空，且数值相同的情况
    #此时才做递归，做下一层的判断
    outside = self.compare(left.left, right.right) #左子树：左、 右子树：右
    inside = self.compare(left.right, right.left) #左子树：右、 右子树：左
    isSame = outside and inside #左子树：中、 右子树：中 （逻辑处理）
    return isSame