leetcode98

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98. 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.
Example 1:

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4
5
Input:
    2
   / \
  1   3
Output: true

Example 2:

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5
6
    5
   / \
  1   4
     / \
    3   6
Output: false

Explanation: The input is: [5,1,4,null,null,3,6]. The root node’s value
is 5 but its right child’s value is 4.

Idea

Recurison inorderly. Or divide and conquer.

Code

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class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        self.order = []
        self.inorder(root)
        
        prev = float('-inf')
        for node in self.order:
            if node.val <= prev:
                return False
            prev = node.val
        
        return True

    def inorder(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if root is None:
            return
        
        self.inorder(root.left)
        self.order.append(root)
        self.inorder(root.right)

# version 2
# class Solution(object):
#     def isValidBST(self, root):
#         """
#         :type root: TreeNode
#         :rtype: bool
#         """
#         res, _, _ = self.helper(root)
#         return res

#     def helper(self, root):
#         """
#         :type root: TreeNode
#         :rtype: bool
#         """
#         if root is None:
#             return True, float('-inf'), float('inf')
#         left, leftmax, leftmin = self.helper(root.left)
#         right, rightmax, rightmin = self.helper(root.right)

#         rootmax = max(root.val, max(leftmax, rightmax))
#         rootmin = min(root.val, min(rightmin, leftmin))
        
#         if left and right:
#             return leftmax < root.val and root.val < rightmin, rootmax, rootmin 

#         return False, float('-inf'), float('inf')