098 Validate Binary Search Tree

Original Description: Given a binary tree, determine if it is a valid binary search tree (BST). Assume a BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node’s key.
  • The right subtree of a node contains only nodes with keys greater than the node’s key.
  • Both the left and right subtrees must also be binary search trees.

  • 要求: 给一个binary tree。

  • 返回: 判断它是不是一棵BST

  • 例子:
    • input:
      2 /
      1 3
    • output: True


利用stack, 从root 走到最左边节点,并依次将经过的节点加入stack,再依次比较左中右。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def isValidBST(self, root):
        :type root: TreeNode
        :rtype: bool
        stack = []
        curr = root
        prev = None
        while curr or stack:
            while curr:
                curr = curr.left  # 从root走到最左边节点,并保留所经过的节点到stack
            if stack:
                curr = stack.pop()
                if prev and curr.val <= prev.val:
                    return False
                prev = curr
                curr = curr.right 
        return True