21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

 

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
  if (l1 == null && l2 == null) {
    return null;
  }
  if (l1 == null) {
    return l2;
  }
  if (l2 == null) {
    return l1;
  }
  ListNode dummy = new ListNode(-1);
  ListNode cur = dummy;
  while (l1 != null && l2 != null) {
    if (l1.val <= l2.val) {
      cur.next = l1;
      l1 = l1.next;
    } else {
      cur.next = l2;
      l2 = l2.next;
    }
    cur = cur.next;
  }
  if (l1 != null) {
    cur.next = l1;
  }
  if (l2 != null) {
    cur.next = l2;
  }
  return dummy.next;
}