Solution to Merge Two Sorted Lists problem.

The first one is recursive solution.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null)
            return l2;
        
        if(l2 == null)
            return l1;
        
        if(l1.val <= l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
            
    }
}

Iterative Solution

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // maintain an unchanging reference to node ahead of the return node.
        ListNode prehead = new ListNode(-1);

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

        // exactly one of l1 and l2 can be non-null at this point, so connect
        // the non-null list to the end of the merged list.
        prev.next = l1 == null ? l2 : l1;

        return prehead.next;
    }
}