题目：

题解：

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = Noneclass Solution(object):def isValidBST(self, root):""":type root: TreeNode:rtype: bool"""pre = -float('inf')p = rootst = []while p is not None or st:while p is not None:st.append(p)p = p.leftp = st.pop()if p.val > pre:pre = p.valelse:return Falsep = p.rightreturn True