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 class Solution {

    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 result = new ListNode(0);

        ListNode prev = result;

        while (l1 != null && l2 != null) {

            if (l1.val <= l2.val) {

                prev.next = l1;

                l1 = l1.next;

            } else {

                prev.next = l2;

                l2 = l2.next;

            }

            prev = prev.next;

        }

        if (l1 != null) {

            prev.next = l1;

        }

        if (l2 != null) {

            prev.next = l2;

        }

        return result.next;

    }

}