卖萌的弱渣

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Validate Binary Search Tree

Validate Binary Search Tree

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. A single node tree is a BST

Example

An example:

1
2
3
4
5

  2
 / \
1   4
   / \
  3   5

The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).

Solution

  • Time O(n):
  • Bring in a recursive helper includes “upper” and “lower”. Check each node if upper > node.val > lower
(Validate-Binary-Search-Tree.py) download
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"""
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 isValidNode(self,node,upper,lower):
        if node == None:
            return True
        if node.val >= upper or node.val <=lower:
            return False

        if node.left != None and self.isValidNode(node.left,node.val,lower) == False:
            return False
        if node.right != None and self.isValidNode(node.right,upper,node.val) == False:
            return False

        return True
    def isValidBST(self, root):
        # write your code here
        # case: {10,5,15,#,#,6,20} require "upper" and "lower" in recursive
        return self.isValidNode(root,sys.maxint,-sys.maxint-1)