21. Merge Two Sorted Lists

https://leetcode.com/problems/merge-two-sorted-lists

Python

(This is be used in 143. Reorder List)

class Solution:
    def mergeTwoLists(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        cur1, cur2 = l1, l2
        head = ListNode(val=-1)
        current = head

        while(cur1 and cur2):
            if cur1.val > cur2.val:
                current.next = cur2
                cur2 = cur2.next
            else:
                current.next = cur1
                cur1 = cur1.next
            current = current.next

        if cur1 is not None:
            current.next = cur1

        if cur2 is not None:
            current.next = cur2

        return head.next

Javascript

 var mergeTwoLists = function(list1, list2) {
  let head = new ListNode();
  let start = head;
  while (list1 !== null && list2 !== null) {
    if (list1.val < list2.val) {
      start.next = list1;
      list1 = list1.next;
    } else {
      start.next = list2;
      list2 = list2.next;
    }
    start = start.next;
  }
  start.next = list1 === null ? list2 : list1;
  return head.next;
};