MergeTwoSortedLists.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class MergeTwoSortedLists {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        while (l1 != null || l2 != null) {
            if (l1 == null) {
                tail.next = l2;
                l2 = l2.next;
            } else if (l2 == null) {
                tail.next = l1;
                l1 = l1.next;
            } else {
                if (l1.val <= l2.val) {
                    tail.next = l1;
                    l1 = l1.next;
                } else {
                    tail.next = l2;
                    l2 = l2.next;
                }
            }
            tail = tail.next;
        }
        return dummy.next;
    }
}