23-Merge-K-Sorted-Lists.js

/**
 * https://leetcode.com/problems/merge-k-sorted-lists/
 * Time O(N) | Space O(N)
 * @param {ListNode[]} lists
 * @return {ListNode}
 */
 var mergeKLists = function(lists) {
    let previous = null;

    for (let i = 0; i < lists.length; i++) {
        previous = mergeTwoLists(previous, lists[i]);
    }

    return previous;
};

var mergeTwoLists = function(list1, list2) {
    let sentinel = tail = new ListNode(0);

    while (list1 && list2) {
        const canAddL1 = list1.val <= list2.val;
        if (canAddL1) {
            tail.next = list1;
            list1 = list1.next;
        } else {
            tail.next = list2;
            list2 = list2.next;
        }

        tail = tail.next;
    }

    tail.next = list1 || list2;

    return sentinel.next;
};

/**
 * https://leetcode.com/problems/merge-k-sorted-lists/
 * Time O(N * log(K)) | Space O(N + K)
 * @param {ListNode[]} lists
 * @return {ListNode}
 */
 var mergeKLists = function(lists) {
    const minHeap = getMinHeap(lists);

    return mergeLists(minHeap)
};

const getMinHeap = (lists) => {
    const heap = new MinPriorityQueue({ priority: ({ val }) => val });

    for (const node of lists) {
        if (!node) continue;

        heap.enqueue(node);
    }

    return heap;
}


const mergeLists = (minHeap) => {
    let sentinel = tail = new ListNode();

    while (!minHeap.isEmpty()) {
        const node = minHeap.dequeue().element;

        tail.next = node;
        tail = tail.next;

        if (!node.next) continue;

        minHeap.enqueue(node.next);
    }

    return sentinel.next;
}