LeetCode-Merge Two Sorted Lists

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

Iterative Approach :

/**
 * 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) {
        ListNode preNode = new ListNode(-1);
        ListNode prev = preNode;
        
        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;
        }
        prev.next = l1 == null ? l2 : l1;
        return preNode.next;
    }
}

Time Complexity : O(n+m) Space Complexity O(1)

Recursive Approach:

/**
 * 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) {
        if(l1 == null){
            return l2;
        }else if(l2 == null){
            return l1;
        }else if(l1.val <l2.val){
                l1.next = mergeTwoLists(l1.next,l2);
            return l1;
            }else{
                l2.next = mergeTwoLists(l1,l2.next);
            return l2;
            }
        }
}

Time Complexity : O(n+m) Space Complexity : O(n+m)