mergeSortedLists.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; }
 * }
 */
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) { //To detect the first break
            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 since they are already sorted lists.
        prev.next = l1 == null ? l2 : l1;

        return prehead.next;
    }
}