Published Feb 23, 2022
[
Given the root of a binary tree, determine if it is a valid binary search tree (BST).
A valid BST is defined as follows:
Input: root = [2,1,3]
Output: true
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.
- The number of nodes in the tree is in the range [1, 104].
- -231 <= Node.val <= 231 - 1
/**
* 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 isValidBST(root: TreeNode | null): boolean {
// each node should satisfy left & right boundary
const valid = (node: TreeNode, left: number, right: number): boolean => {
// base case
if(node === null) {
return true;
}
if(!(left < node.val && node.val < right)) {
return false;
}
// recursive case
return valid(node.left, left, node.val) && valid(node.right, node.val, right)
}
return valid(root, -Infinity, Infinity)
};