17、树的子结构

输入两棵二叉树A，B，判断B是不是A的子结构。（ps：我们约定空树不是任意一个树的子结构）

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def HasSubtree(self, pRoot1, pRoot2):
        # write code here
        if not pRoot1 or not pRoot2:
            return False
        return self.helper(pRoot1, pRoot2) or self.helper(pRoot1.left, pRoot2) or self.helper(pRoot1.right, pRoot2)
    def helper(self, p1, p2):
        if not p2:
            return True
        if not p1 and p2:
            return False
        if p1.val != p2.val:
            return False
        return self.helper(p1.left, p2.left) and self.helper(p1.right, p2.right)