Merge Two Linked Lists

Interviewer Prompt

Write a function that takes in the heads of two Singly Linked Lists that are in sorted order, respectively. The function should merge the lists in place (i.e., it shouldn't create a brand new list) and return the head of the merged list; the merged list should be in sorted order.

Each LinkedList node has an integer value as well as a next node pointing to the next node in the list or to null if it's the tail of the list.

Examples

headOne = 2 -> 6 -> 7 -> 8 // the head node with value 2
headTwo = 1 -> 3 -> 4 -> 5 -> 9 -> 10 // the head node with value 1
mergeLinkedLists(headOne, headTwo) = 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10 // the new head node with value 1

Interviewer Strategy Guide

First, make sure the interviewee understands the structure of a LinkedList and its constraints. Because the lists are already sorted, traditional sorting algorithms won't be needed.

The interviewee can implement this algorithm either iteratively or recursively following nearly identical logic, so you can leave that up to them.

Most interviewees will get the process, but it's very easy to "solve" the problem by creating a malformed LinkedList. Make sure 3 nodes are being manipulated at every step to ensure the final LinkedList doesn't have any holes or misdirected pointers.

Hints

• You can iterate through the Linked Lists from head to tail and merge them along the way by inserting nodes from the second Linked List into the first Linked List.
• You'll need to manipulate three nodes at once at every step.

Solutions

Optimized Solution 1: O(n+m) time complexity, O(1) space complexity (n=length(headOne),m=length(headTwo))

function mergeLinkedLists(headOne, headTwo) {
  let p1 = headOne;
  let p1Prev = null;
  let p2 = headTwo;
  while (p1 !== null && p2 !== null) {
    if (p1.value < p2.value) {
      p1Prev = p1;
      p1 = p1.next;
    } else {
      if (p1Prev !== null) p1Prev.next = p2;
      p1Prev = p2;
      p2 = p2.next;
      p1Prev.next = p1;
    }
  }
  if (p1 === null) p1Prev.next = p2;
  return headOne.value < headTwo.value ? headOne : headTwo;
}

Optimized Solution 2: O(n+m) time complexity, O(n+m) space complexity (n=length(headOne),m=length(headTwo))

function mergeLinkedLists(headOne, headTwo) {
  recursiveMerge(headOne, headTwo, null);
  return headOne.value < headTwo.value ? headOne : headTwo;
}
​
function recursiveMerge(p1, p2, p1Prev) {
  if (p1 === null) {
    p1Prev.next = p2;
    return;
  }
  if (p2 === null) return;

​  if (p1.value < p2.value) {
    recursiveMerge(p1.next, p2, p1);
  } else {
    if (p1Prev !== null) p1Prev.next = p2;
    const newP2 = p2.next;
    p2.next = p1;
    recursiveMerge(p1, newP2, p2);
  }
}