参考代码:

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None


class Solution:
    def help(self, p, q):
        if p == None and q == None:
            return True
        if p and q and p.val == q.val:
            return self.help(p.right, q.left) and self.help(p.left, q.right)
        return False

    def isSymmetric(self, root):
        if root:
            return self.help(root.right, root.right)
        return True