【LeetCode】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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

Solution:

Create a dummy node as the head of the new list, keep comparing val of node l1 and node l2, append the smaller one to new list.

Code:

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

Code 2: same idea with code improved

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