二叉树问题---判断一棵树是否为搜索二叉树

#判断一棵树是否为搜索二叉树def isBST(root):    if not root:        return True    res = True    pre = None    cur1 = root    cur2 = None    while cur1:        cur2 = cur1.left        if cur2:            while cur2.right and cur2.right != cur1:                cur2 = cur2.right            if cur2.right == None:                cur2.right = cur1                cur1 = cur1.left                continue            else:                cur2.right = None        if pre and int(pre.val) > int(cur1.val):            res = False        pre = cur1        cur1 = cur1.right    return res