Merge Sorted Lists

@interface ListNode: NSObject
@property (nonatomic) ListNode* next;
@property (nonatomic) NSNumber *data;
@end

@implementation ListNode
@end

@interface Solution: NSObject
-(ListNode *)sortList:(ListNode *)l1 andList:(ListNode *)l2;
@end

@implementation Solution
-(ListNode *)sortList:(ListNode *)l1 andList:(ListNode *)l2 {
    if(l1 == nil) return l2;
    if(l2 == nil) return l1;
    ListNode *sortedTail = [ListNode new];
    ListNode *head = sortedTail;
    while(l1 && l2) {
        if([l1.data compare:l2.data] == NSOrderedAscending) {
            sortedTail.next = l1;
            sortedTail = l1;
            l1 = l1.next;
        } else {
            sortedTail.next = l2;
            sortedTail = l2;
            l2 = l2.next;
        }
        sortedTail.next = nil;
    }
    if(l1) sortedTail.next = l1;
    else sortedTail.next = l2;

    return head.next;
}
@end