如果一棵二叉树和它的镜像一样，那么它是镜像的。

class Solution:
    def isSymmetrical(self, pRoot):
        # write code here
        return self.Symmetrical(pRoot, pRoot)
    def Symmetrical(self, pRoot1, pRoot2):
        if not pRoot1 and not pRoot2:
            return True
        if not pRoot1 or not pRoot2:
            return False
        if pRoot1.val != pRoot2.val:
            return False
        return self.Symmetrical(pRoot1.left, pRoot2.right) and  self.Symmetrical(pRoot1.right, pRoot2.left)