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.

Iterative

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

public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null)
return l2;
if (l2 == null)
return l1;

ListNode dummy = new ListNode(0);
ListNode cur = dummy;
while (l1 != null && l2 != null){
if (l1.val < l2.val){
cur.next = l1;
l1 = l1.next;
}else{
cur.next = l2;
l2 = l2.next;
}
cur = cur.next;
}
if (l1 != null){
cur.next = l1;
}else{
cur.next = l2;
}
return dummy.next;
}
}

Recursive

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public 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;
}
}
}