moonlightshadow123
4/10/2017 - 3:36 PM

98-Validate-Binary-Search-Tree https://leetcode.com/problems/validate-binary-search-tree/

98-Validate-Binary-Search-Tree https://leetcode.com/problems/validate-binary-search-tree/

inorder_trav.py
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# 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
        """
        self.prev = None
        return self.isMonotonicIncreasing(root)
    
    def isMonotonicIncreasing(self, node):
        if node == None:
            return True
        if not self.isMonotonicIncreasing(node.left):
            return False
        if self.prev != None and self.prev.val >= node.val:
            return False
        self.prev = node
        if not self.isMonotonicIncreasing(node.right):
            return False
        return True
high_low_bound.py
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# 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
        """
        return self.recursive(-float('inf'), root, float('inf'))
    def recursive(self, lowBound, node, highBound):
        if node == None:
            return True
        return node.val < highBound and node.val > lowBound and\
               self.recursive(lowBound, node.left, node.val) and self.recursive(node.val, node.right, highBound)
98-Validate-Binary-Search-Tree.md
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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. Example 1:
    2
   / \
  1   3

Binary tree [2,1,3], return true. Example 2:

    1
   / \
  2   3

Binary tree [1,2,3], return false.