20块钱，1块钱1瓶，两个空瓶子可以换一瓶，问最多可以喝几瓶？

            //方法一
            int money = 20;
            int pingzi = 0;
            int total = 0;
            while (money > 0 || pingzi > 1)
            {
                total++;
                if (money > 0)
                {
                    money--;
                    pingzi++;
                }
                else if (pingzi > 1)
                {
                    pingzi--;
                }
            }
            Console.WriteLine(total.ToString());

 //方法二
            int money = 20;
            int bottle = money;
            int sum = bottle;
            while (bottle > 1)
            {
                sum += bottle / 2;
                bottle = bottle / 2 + bottle%2;
            }
            Console.WriteLine(sum.ToString());

 //方法三 迭代
            int a = drink(20);
            Console.WriteLine(a);

           

private static int drink(int n)
        {
            if (n == 1)
                return 1; //如果只有一瓶水了，不可以借瓶子，就只能喝这一瓶 return 1，如果可以借，就喝两瓶，return 2;
            return 2 + drink(n - 1);
        }