0098-validate-binary-search-tree.ts

/**
 * 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 {
    return validate(root, null, null);
}

function validate(
    root: TreeNode | null,
    max: number | null,
    min: number | null
): boolean {
    if (!root) {
        return true;
    }

    if (
        (max !== null && root.val >= max) ||
        (min !== null && root.val <= min)
    ) {
        return false;
    }

    return (
        validate(root.left, root.val, min) &&
        validate(root.right, max, root.val)
    );
}