json与NSString转换

json to string

NSData *jsonData = [NSJSONSerialization dataWithJSONObject:json
                                                   options:NSJSONWritingPrettyPrinted
                                                     error:nil];
NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];

string to json

NSData *stringData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
id json = [NSJSONSerialization JSONObjectWithData:stringData options:0 error:nil];