Merge Two Sorted Lists

Merge Two Sorted Lists - LeetCodeLeetCode

Problem

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Pseudocode

- traverse both lists simutaneously using a while loop
- create a new LL, linking it a copy of the list with smallest value
    - advance BOTH 
        - linked list with the smaller value
        - new LL
    - if either one of the LL is null, attach the non-null LL to the new LL
    - while loop terminates when both LLs are null
- return new LL.next

Input: list1 = [1,2,4], list2 = [1,3,4]
[0]
[0,1,3,4]
[0,1,1,2,4]
[0,1,1,2,4]
[0,1,1,2,3,4]
[0,1,1,2,3,4]
[0,1,1,2,3,4,4]

return result.next
[1,1,2,3,4,4]]

Solution

const mergeTwoLists = (l1, l2) => {
  const result = new ListNode(null);
  let n = result;

  while (l1 !== null || l2 !== null) {
    if (l1 === null) {
      n.next = l2;
      l2 = l2.next;
    } else if (l2 === null) {
      n.next = l1;
      l1 = l1.next;
    } else if (l1.val < l2.val) {
      n.next = l1;
      l1 = l1.next;
    } else {
      n.next = l2;
      l2 = l2.next;
    }
    n = n.next;
  }

  return result.next;
};

Time and Space Complexity

Time

• traversing the longest sorted linked list - O(N)

• assigning node.next to new list - O(1)

• Total - O(N)

Space

• creating a new sorted linked list, sum of both input - O(N)

• Total - O(N)