Interview Question: Merge two sorted singly linked lists without creating new nodes

Question

This is a programming question asked during a written test for an interview. \"You have two singly linked lists that are already sorted, you have to merge them and return a

Answer 1

Recursion should not be needed to avoid allocating a new node:

Node MergeLists(Node list1, Node list2) {
  if (list1 == null) return list2;
  if (list2 == null) return list1;

  Node head;
  if (list1.data < list2.data) {
    head = list1;
  } else {
    head = list2;
    list2 = list1;
    list1 = head;
  }
  while(list1.next != null) {
    if (list1.next.data > list2.data) {
      Node tmp = list1.next;
      list1.next = list2;
      list2 = tmp;
    }
    list1 = list1.next;
  } 
  list1.next = list2;
  return head;
}

Answer 2

Simple code in javascript to merge two linked list inplace.

function mergeLists(l1, l2) {
    let head = new ListNode(0); //dummy
    let curr = head;
    while(l1 && l2) {
        if(l2.val >= l1.val) {
            curr.next = l1;
            l1 = l1.next;
        } else {
            curr.next = l2;
            l2=l2.next
        }
        curr = curr.next;
    }
    if(!l1){
        curr.next=l2;
    }
    if(!l2){
        curr.next=l1;
    } 
    return head.next;
}