public class LeetCode0021 {

    static class ListNode {
        int val;
        ListNode next;
        ListNode(int x) { val = x; }
    }

    public ListNode mergeTwoLists(ListNode l1, ListNode l2)
    {

        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        ListNode head = new ListNode(0);
        ListNode p = head;

        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                p.next = l1;
                l1 = l1.next;
            }
            else {
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }

        p.next = l1 == null ? l2 : l1;

        return head.next;
    }
}

Add LeetCode0021.java