MergeTwoSortedLists21.java

/**
 * 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
 *
 */

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

public class MergeTwoSortedLists21 {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode p = dummy;
        while (l1 != null && l2 != null) {
            if (l1.val >= l2.val) {
                p.next = l2;
                l2 = l2.next;
            } else {
                p.next = l1;
                l1 = l1.next;
            }
            p = p.next;
            p.next = null;
        }
        if (l1 != null) p.next = l1;
        if (l2 != null) p.next = l2;
        return dummy.next;
    }

    public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;

        if (l1.val >= l2.val) {
            l2.next = mergeTwoLists2(l1, l2.next);
            return l2;
        } else {
            l1.next = mergeTwoLists2(l1.next, l2);
            return l1;
        }
    }

}