树的子结构

class Solution:
    def HasSubtree(self, a, b):
        # write code here
        if not b or not a:
            return False

        return self.contains(a, b) or self.HasSubtree(a.left, b) or self.HasSubtree(a.right, b)

    def contains(self, a, b):
        if not b:
            return True
        if not a:
            return False

        return a.val == b.val and self.contains(a.left, b.left) and self.contains(a.right, b.right)