new_contents: """
  Definition of TreeNode:
  class TreeNode:
      def __init__(self, val):
          self.val = val
          self.left, self.right = None, None
  """
  class Solution:
      """
      @param root: The root of binary tree.
      @return: True if the binary tree is BST, or false
      """  
      def isValidBST(self, root):
          # write your code here
          stack, pre = [root], None
          while stack != [] and stack[0] is not None:
              node = stack.pop()
              while node is not None:
                  stack.append(node)
                  node = node.left
              node = stack.pop()
              if pre is not None and pre.val >= node.val:
                  return False
              pre = node
              stack.append(node.right)
          return True
  
subject: Create ValidateBST_iter.py