IOS 将请求参数字典转换为参数字符串

- (NSString *)URLRequestStringWithURL:(NSString *)urlstr{

NSMutableString *URL = [NSMutableString stringWithFormat:@"%@",urlstr];
//获取字典的所有keys
NSArray * keys = [self allKeys];

//拼接字符串
for (int j = 0; j < keys.count; j ++){
NSString *string;
if (j == 0){
//拼接时加？
string = [NSString stringWithFormat:@"?%@=%@", keys[j], self[keys[j]]];

}else{
//拼接时加&
string = [NSString stringWithFormat:@"&%@=%@", keys[j], self[keys[j]]];
}
//拼接字符串
[URL appendString:string];

}
return URL;
}