# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param root TreeNode类 
# @return int整型一维数组
#
class Solution:
    def isValidBST(self , root ):
        pre_node = None
        stack = []
        while root:
            stack.append(root)
            root = root.left
        while stack:
            top = stack.pop()
            if not pre_node:
                pre_node = top
            else:
                if pre_node.val>=top.val:
                    return False
                else:
                    pre_node = top
            right = top.right
            while right:
                stack.append(right)
                right = right.left
        return True