21. Merge Two Sorted Lists in JavaScript (Leetcode Problem)

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.

Example 1:

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

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

Solution

Space Complexity Constant, Time complexity O(n)

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function(list1, list2) {
  const dummynode = new ListNode(-Infinity);
  let prev = dummynode;
  while(list1 && list2){
      if(list1.val <= list2.val){
          prev.next = list1;
          prev = list1;
          list1 = list1.next;
      }
      else{
          prev.next = list2;
          prev = list2;
          list2 = list2.next;
      }
  }
  if(!list1){
      prev.next = list2;
  }
  if(!list2){
      prev.next = list1;
  }
    return dummynode.next;
};