算法面试题

1. C# 1到100,每三个数 ,取第三个数,直到输出最后一个数。

注释：比如1-100，第一次输出的数就是3，6，9，12，15，18.....99，第二次输出的就是9，18，27，36，45，....99，直到最后输出为一个数。

别人的写法：

class Program
    {
        static void Main(string[] args)
        {
            List<int> list = new List<int>();
            InitList(list);
            RemoveIt(ref list);
            for (int i = 0; i < list.Count; i++)
                Console.WriteLine(list[i]);
            Console.ReadLine();
        }
        static void InitList(List<int> list)
        {
            for (int i = 1; i <= 100; i++)
                list.Add(i);
        }
        static void RemoveIt(ref List<int>  list)
        {
            List<int> result = new List<int>();
            for (int i = 1; i <= list.Count; i++)
            {
                if (i % 3 == 0)
                {
                    result.Add(list[i - 1]);
                    if (result.Count > 1) Console.Write(","); 
                    Console.Write(list[i - 1]);
                }
            }
            Console.WriteLine();
            list = result;
            if (list.Count >= 3) RemoveIt(ref list);
        }
    }

自己的写法：

 class Program
    {
        static void Main(string[] args)
        {
            Dictionary<int, int> dic = new Dictionary<int, int>();
            for (int i = 1; i < 101; i++)
            {
                dic.Add(i, i);
            }

            while (true)
            {
                dic = run(dic);
                if (dic.Count == 1)
                {
                    break;
                }
            }
            Console.ReadKey();
        }

        private static Dictionary<int, int> run(Dictionary<int, int> num)
        {
            int count = 0;
            int dicIndex = 1;
            Dictionary<int, int> dic = new Dictionary<int, int>();
            foreach (var item in num)
            {
                count++;
                if (count == 3)
                {
                    count = 0;
                    dic.Add(dicIndex, item.Value);
                    dicIndex++;
                }
            }

            //打印验证数据准确性
            foreach (var item in dic)
            {
                Console.WriteLine(item.Key + ":" + item.Value);
            }
            Console.WriteLine("\r\n");

            return dic;
        }
    }

2.54张牌洗牌算法：

    class Program
    {
        static void Main(string[] args)
        {
            int[] numbers = new int[54];
            int length = numbers.Length;
            for (int i = 0; i < length; i++)
            {
                numbers[i] = i + 1;
            }

            random(numbers, length);
            foreach (int i in numbers)
            {
                Console.Write(i + " ");
            }
            Console.ReadKey();
        }

        private static void random(int[] array, int length)
        {
            int index;
            int value;
            for (int i = length - 1; i > 0; i--)
            {
                index = new Random().Next(0, i + 1);

                value = array[i];
                array[i] = array[index];
                array[index] = value;
            }
        }
    }

3.数据由小到大排序

一般的算法

      int[] nums = new int[] { 56, 2, 65, 36, 21, 3, 5, 4, 6, 54, 88 };
            for (int i = 0; i < nums.Length - 1; i++)
            {
                for (int j = 0; j < nums.Length - 1 - i; j++)
                {
                    if (nums[j] > nums[j + 1])
                    {
                        int temp = nums[j];
                        nums[j] = nums[j + 1];
                        nums[j + 1] = temp;
                    }
                }
            }
            foreach (var n in nums)
            {
                Console.WriteLine(n);
            }
            Console.ReadKey();

用Linq也可以实现

 int[] nums = new int[] { 56, 2, 65, 36, 21, 3, 5, 4, 6, 54, 88 };
            var result = from c in nums.ToList()
                         orderby c 
                         select c;
            foreach (var n in result)
            {
                Console.WriteLine(n);
            }

            Console.ReadKey();

以上两种最终实现的效果一样。