LeetCode 21: Merge Two Sorted Lists

/**
 * 21. Merge Two Sorted Lists
 * 1. Time:O(min(m,n)) Space:O(1)
 * 2. Time:O(m+n) Space:O(m+n)
 */

// 1. Time:O(min(m,n)) Space:O(1)
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 prev = dummy;
        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 dummy.next;
    }
}

// 2. Time:O(m+n) Space:O(m+n)
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;
        }
    }
}