some basics using NSJSONSerialization (iOS5 and up.)

gistfile1.m

NSString *strArrayExample = @"%5B%22hello%22%2C%22world!%22%2C12%5D";
NSString *strObjectExample = @"%7B%22a%22%3A%221%22%2C%22b%22%3A%22c%22%7D";

NSString *str = [strArrayExample stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSLog(@"str:: %@", str);

NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];
id json = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];
NSLog(@"type of data:: %@", NSStringFromClass([json class]));

if ([json isKindOfClass:[NSArray class]]) {
	NSLog(@"is array");
} else if ([json isKindOfClass:[NSDictionary class]]) {
	NSLog(@"is object");
}