Merge Two Sorted Lists

MergeTwoSortedListsIterative.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.
 */
 /**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode curr = head;
        while(l1 != null && l2 != null){
            if(l1.val <= l2.val){
                curr.next = l1;
                l1 = l1.next;
            }else{
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }
        if(l1 != null)
            curr.next = l1;
        else if(l2 != null)
            curr.next = l2;
        return head.next;
    }
}

MergeTwoSortedListsRecursion.java

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null)
            return l2;
        if(l2 == null)
            return l1;
        ListNode head = new ListNode(0);
        if(l1.val <= l2.val){
            head = l1;
            head.next = mergeTwoLists(l1.next, l2);
        }else{
            head = l2;
            head.next = mergeTwoLists(l2.next, l1);
        }
        return head;
    }
}