LeetCode in TypeScript
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'
/**
* Definition for a binary tree node.
* class TreeNode {
* val: number
* left: TreeNode | null
* right: TreeNode | null
* constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
* }
*/
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 }