Merge two sorted (ascending) linked lists and return it as a new sorted list. The new sorted list should be made by splicing together the nodes of the two lists and sorted in ascending order.

Given 1->3->8->11->15->null, 2->null , return 1->2->3->8->11->15->null

Key idea:

1. Use a dummy node to simplify head node management;

2. Apply the merge procedure in merge sort.

 1 /**
 2  * Definition for ListNode.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int val) {
 7  *         this.val = val;
 8  *         this.next = null;
 
 9  *     }
10  * }
11  */
12 
13 
14 public class Solution {
15     /*
16      * @param l1: ListNode l1 is the head of the linked list
17      * @param l2: ListNode l2 is the head of the linked list
18      * @return: ListNode head of linked list
19      */
20     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
 
21         if(l1 == null) {
22             return l2;
23         }
24         else if(l2 == null) {
25             return l1;
26         }
27         ListNode dummy = new ListNode(0);
28         ListNode curr = dummy, curr1 = l1, curr2 = l2, temp = null;
29         while(curr1 != null && curr2 != null) {
30             if(curr1.val <= curr2.val) {
31                 curr.next = curr1;
32                 temp = curr1.next;
33                 curr1.next = null;
34                 curr1 = temp;
35             }
36             else {
37                 curr.next = curr2;
38                 temp = curr2.next;
39                 curr2.next = null;
40                 curr2 = temp;
41             }
42             curr = curr.next;
43         }
44         if(curr1 != null) {
45             curr.next = curr1;
46         }
47         else if(curr2 != null) {
48             curr.next = curr2;
49         }
50         return dummy.next;
51     }
52 }

Related Problems

Merge Two Sorted Arrays

[LintCode] Merge Two Sorted Lists