reverse.m

#import <Foundation/Foundation.h>
#import <stdio.h>

@class Node;

@interface Node : NSObject
@property (strong) Node* next;
@property (assign) NSString* val;
@end
  
@implementation Node
+ (id)nodeWithVal:(NSString*)val
{
  Node* tmp = [[Node alloc] init];
  tmp.val = val;
  return tmp;
}
@end

Node* reverse1(Node* list)
{
  Node* prev = nil;
  while (list) {
    Node* next = list.next;
    list.next = prev;
    prev = list;
    list = next;
  }
  return prev;
}

Node* reverse2(Node* list)
{
  Node* r = nil;
  while (list) {
    Node* cur = list;
    list = list.next;
    cur.next = r;
    r = cur;
  }
  return r;
}

void dump(Node* list)
{
for (Node* cur=list; cur!=nil; ) {
      printf("%s", [cur.val UTF8String]);
      cur=cur.next;
      if (cur) {
        printf(" -> ");
      }
    }
  printf("\n");
}

int main (int argc, const char * argv[])
{
  @autoreleasepool {
    Node* list = [Node nodeWithVal:@"a"];
    list.next = [Node nodeWithVal:@"b"];
    list.next.next = [Node nodeWithVal:@"c"];    
    list.next.next.next = [Node nodeWithVal:@"d"];
    
    dump(list);
    list = reverse1(list);
    dump(list);
    list = reverse2(list);
    dump(list);

  }
}

// OUTPUT:
// running 72 lines of Objective-C
// a -> b -> c -> d
// d -> c -> b -> a
// a -> b -> c -> d