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