Categories for NSDictionary and NSArray for swapping the keys of a dictionary for it's value.

Swaping (key-value)

@interface NSDictionary (Swapping)

- (NSDictionary *)dictionaryBySwappingKeyWithValue;

@end

@interface NSDictionary (Swapping)

- (NSDictionary *)dictionaryBySwappingKeyWithValue
{
    NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionaryWithCapacity:self.count];

    [self enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL *stop) {
        id newKey = nil;
        if ([value isKindOfClass:[NSDictionary class]]) {
            newKey = [value dictionaryBySwappingKeyWithValue];

        } else if ([value isKindOfClass:[NSArray class]]) {
            newKey = [value arrayBySwappingKeyWithValue];
        } else {
            newKey = value;
        }

        if (![newKey conformsToProtocol:@protocol(NSCopying)]) {
            newKey = [NSValue valueWithNonretainedObject:newKey];
        }

        mutableDictionary[newKey] = key;
    }];

    return [NSDictionary dictionaryWithDictionary:mutableDictionary];
}

@end

@interface NSArray (Swapping)

- (NSArray *)arrayBySwappingKeyWithValue;

@end

@implementation NSArray (Swapping)

- (NSArray *)arrayBySwappingKeyWithValue
{
    NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:self.count];

    [self enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        if ([obj isKindOfClass:[NSDictionary class]]) {
            NSDictionary *newDict = [obj dictionaryBySwappingKeyWithValue];
            mutableArray[idx] = newDict;
        } else if ([obj isKindOfClass:[NSArray class]]) {
            NSArray *newArray = [obj arrayBySwappingKeyWithValue];
            mutableArray[idx] = newArray;
        } else {
            mutableArray[idx] = obj;
        }
    }];

    return [NSArray arrayWithArray:mutableArray];
}

@end