MergeTwoLists.java

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

public class MergeTwoLists {

    public static ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                list1.next = mergeTwoLists(list1.next, list2);
                return list1;
            } else {
                list2.next = mergeTwoLists(list1, list2.next);
                return list2;
            }
        }
        if (list1 == null)
            return list2;
        return list1;
    }

    public static void printList(ListNode node) {
        while (node != null) {
            System.out.print(node.val + " ");
            node = node.next;
        }
        System.out.println();
    }

    public static void main(String[] args) {
        ListNode list1 = new ListNode(1);
        list1.next = new ListNode(2);
        list1.next.next = new ListNode(4);

        ListNode list2 = new ListNode(1);
        list2.next = new ListNode(3);
        list2.next.next = new ListNode(4);

        ListNode mergedList = mergeTwoLists(list1, list2);

        System.out.print("Merged List: ");
        printList(mergedList);
    }
}

//time complexity -O(n)
//space complexity -O(1)