LeetCode Validate Binary Search Tree

LeetCode Validate Binary Search Tree
LeetCode-based Validate Binary Search Tree

Original question

Determine whether a binary search tree is valid. Effective means that each node has a value greater than or equal to the left node (if there is a corresponding node), and its left and right nodes also meet this condition.

Note:

None

Example:

Input:

  2 / \1   3

Output: True

Solutions

The Binary Tree Inorder Traversal is modified. In the middle-order traversal of the tree, the node sequence is left node, root node, and right node. This means that when a binary search tree meets the requirements, its ordinal traversal sequence must increase progressively. If the preceding node is larger than the following node in the middle-order traversal, it indicates that it does not meet the requirements.

AC Source Code

# Definition for a binary tree node.class TreeNode(object):    def __init__(self, x):        self.val = x        self.left = None        self.right = Noneclass Solution(object):    def isValidBST(self, root):        """        :type root: TreeNode        :rtype: bool        """        stack = []        curr = root        prev = None        while curr or stack:            while curr:                stack.append(curr)                curr = curr.left            if stack:                curr = stack.pop()                if prev and curr.val <= prev.val:                    return False                prev = curr                curr = curr.right        return Trueif __name__ == "__main__":    None