var assert = require('assert');

​

function mergeSortedLists(head1, head2) {

  // fill in this method

  return head;

}

​

// Supporting data structures

​

function Node(val) {

  this.val = val;

  this.next = null;

}

​

class LinkedList {

  constructor() {

    this.head = null;

    this.tail = null;

  }

​

  prepend(newVal) {

    const currentHead = this.head;

    const newNode = new Node(newVal);

    newNode.next = currentHead;

    this.head = newNode;

​

    if (!this.tail) {