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98. Validate Binary Search Tree

Medium

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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:

Input: root = [2,1,3]

Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]

Output: false

Explanation: The root node’s value is 5 but its right child’s value is 4.

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• -231 <= Node.val <= 231 - 1

Solution

import { TreeNode } from '../../com_github_leetcode/treenode'

function dfs(node: TreeNode | null, lowerBound: number, upperBound: number): boolean {
    if (!node) return true
    if (node.val <= lowerBound) return false
    if (node.val >= upperBound) return false
    return dfs(node.left, lowerBound, node.val) && dfs(node.right, node.val, upperBound)
}

function isValidBST(root: TreeNode | null): boolean {
    return dfs(root, -Infinity, Infinity)
}

export { isValidBST }

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