Merge k Sorted Lists

const push = (heap, item) => {
    if (item === null) return;
    let index = heap.length;
    const { val } = item;
    while (index !== 0) {
        let parent_index = (index - 1) >> 1;
        const parent = heap[parent_index];
        if (parent.val <= val) break;
        heap[index] = parent;
        index = parent_index;
    }
    heap[index] = item;
};
const updateRoot = (heap) => {
    const [ item ] = heap;
    const { val } = item;
    const { length } = heap;
    let index = 0;
    for (;;) {
        let child_index = (index << 1)|1;
        if (child_index >= length) break;
        let child = heap[child_index];
        let other_index = child_index + 1;
        if (other_index < length && heap[other_index].val < child.val) {
            child = heap[other_index];
            child_index = other_index;
        }
        if (child.val >= val) break;
        heap[index] = child;
        index = child_index;
    }
    heap[index] = item;
};
const pop = (heap) => {
    const node = heap[0];
    const { next } = node;
    if (next !== null) {
        heap[0] = next;
        updateRoot(heap);
    } else if (heap.length === 1) {
        heap.length = 0;
    } else {
        const last = heap[heap.length - 1];
        heap.length--;
        heap[0] = last;
        updateRoot(heap);
    }
    return node;
};
const mergeKLists = (lists) => {
    const heap = [];
    for (let list of lists) push(heap, list);
    if (heap.length === 0) return null;
    const list = pop(heap);
    let foot = list;
    while (heap.length) {
        const node = pop(heap);
        foot.next = node;
        foot = node;
    }
    foot.next = null;
    return list;
};