98-Validate-Binary-Search-Tree.js

/**
 * https://leetcode.com/problems/validate-binary-search-tree/
 * Time O(N) | Space O(H)
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function(root, min = -Infinity, max = Infinity) {
    const isBaseCase = root === null;
    if (isBaseCase) return true;

    const isInvalid = (root.val <= min) || (max <= root.val);
    if (isInvalid) return false;

    return dfs(root, min, max);
};

const dfs = (root, min, max) => {
    const left = isValidBST(root.left, min, root.val);
    const right = isValidBST(root.right, root.val, max);

    return left && right;
}
// TODO
/**
 * https://leetcode.com/problems/validate-binary-search-tree/
 * Time O(N) | Space O(H)
 * @param {TreeNode} root
 * @return {boolean}
 */
 var isValidBST = function(root, prev = [ null ]) {
    const isBaseCase = root === null;
    if (isBaseCase) return true;

    if (!isValidBST(root.left, prev)) return false;

    const isInvalid = (prev[0] !== null) && (root.val <= prev[0]);
    if (isInvalid) return false;

    prev[0] = root.val;

    return isValidBST(root.right, prev);
}

/**
 * https://leetcode.com/problems/validate-binary-search-tree/
 * Time O(N) | Space O(H)
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function(root, stack = []) {
    let prev = null;

    while (stack.length || root) {
        moveLeft(stack, root);
        root = stack.pop();

        const isInvalid = prev && (root.val <= prev.val);
        if (isInvalid) return false;

        prev = root;
        root = root.right;
    }

    return true;
}

const moveLeft = (stack, root) => {
    while (root) {
        stack.push(root);
        root = root.left;
    }
}