no. 21: merge two sorted lists from l33tcode (with two approaches: arrays and linked lists)

mergeLinkedLists.js

class Node {
  constructor(val=0, next=null) {
    this.val = val;
    this.next = next;
  }
}

const generateLinkedList = (arr) => {
  let current = new Node();
  const head = current;

  // the following one-liner is the same as:
  //   for (let i = 0; i < arr.length; i++) {
  //     current.next = new Node(arr[i]);
  //     current = current.next;
  //   }
  for (val of arr) current = current.next = new Node(val);

  return head.next;
};

const copyLinkedList = (linkedList, current = new Node(), head = null) => {
  if (!linkedList) return head && head.next;

  if (!head) head = current;

  current = current.next = new Node(linkedList.val);
  return copyLinkedList(linkedList.next, current, head);
};

// solve using arrays
const mergeArrays = (a1, a2) => {
  // declare copies so as to preserve original arrays
  let arr1 = [...a1];
  let arr2 = [...a2];

  const merged = [];
  let lowerVal;

  while (arr1.length || arr2.length) {
    if (!arr1.length) return [...merged, ...arr2];
    if (!arr2.length) return [...merged, ...arr1];

    if (arr1[0] < arr2[0]) {
      lowerVal = arr1.shift(); // removes and returns first element from array
    } else {
      lowerVal = arr2.shift();
    }

    merged.push(lowerVal)
  }

  return merged;
};

// solve using linked lists
const mergeLinkedLists = (l1, l2) => {
  // declare copies so as to preserve original linked lists
  let list1 = copyLinkedList(l1);
  let list2 = copyLinkedList(l2);

  let current = new Node();
  const head = current;

  while (list1 || list2) {
    if (!list1) {
      current.next = list2;
      break;
    } else if (!list2) {
      current.next = list1;
      break;
    } else if (list1.val < list2.val) {
      current = current.next = list1;
      list1 = list1.next;
    } else {
      current = current.next = list2;
      list2 = list2.next;
    }
  }

  return head.next;
};

// ********** TEST DATA BELOW **********

// sample lists
const array1 = [1,1,1,11,728]; //[1, 3, 5, 7];
const array2 = [1,2,3,3,4,103,1002]; //[2, 4, 6];

// generate merged arrays
const mergedArrays = mergeArrays(array1, array2);

console.log('merged array values:');
for (val of mergedArrays) console.log(val); // print values from merged arrays
console.log(); // add a blank line

// generate merged linked list
const linkedList1 = generateLinkedList(array1);
const linkedList2 = generateLinkedList(array2);
const mergedLinkedLists = mergeLinkedLists(linkedList1, linkedList2);

// print values from merged linked lists
console.log('merged linked list values:');
let mergedCopy = mergedLinkedLists;
while (mergedCopy) {
  console.log(mergedCopy.val);
  mergedCopy = mergedCopy.next;
}