求：算法

Question

List<string>  num = new List<string> {"1","2","3","4","5","6","7"};首先，任意挑选两个出来，再从剩下的挑选两个出来比如：12 - 34
12 - 35
12 - 36
12 - 37
12 - 45
12 - 46
12 - 47
12 - 56
12 - 57
12 - 67总共 21*10 = 210注

Answer 1

C74 = 35C72*C52 = 210

Answer 2

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "1234567";
            List<string> list = Combo(s, 4).SelectMany(x => Arrange(x)).ToList();
            Console.WriteLine("count: " + list.Count);
            list.ForEach(x => Console.WriteLine(x.Substring(0, 2) + " - " + x.Substring(2, 2)));
        }        static IEnumerable<string> Arrange(string source)
        {
            for (int i = 0; i < source.Length; i++)
            {
                if (source.Length == 1)
                {
                    yield return source;
                }
                else
                {
                    foreach (var x in Arrange(source.Substring(0, i) + source.Substring(i + 1)))
                    {
                        yield return source[i] + x;
                    }
                }
            }
        }        static IEnumerable<string> Combo(string source, int len)
        {
            int[] pos = new int[len];
            for (int i = 0; i < len; i++) pos[i] = i;
            while (pos[0] < source.Length - len)
            {
                string str = "";
                for (int i = 0; i < len; i++) str += source[pos[i]];
                for (int i = len - 1; i >= 0; i--)
                {
                    if (pos[i] < source.Length - len + i)
                    {
                        pos[i]++;
                        for (int j = i + 1; j <= len - 1; j++)
                        {
                            pos[j] = pos[i] + j - i;
                        }
                        break;
                    }
                    else
                    {
                        continue;
                    }
                }
                yield return str;
            }
            yield return source.Substring(source.Length - len);
        }
    }
}
你怎么算的，一共840组。

Answer 3

先从7个里挑选2个 C72
从剩下的5个里挑选2个，C52,不对吗？C72*C52 = 210 ,不对吗？

Answer 4

210个：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "1234567";
            List<string> list = Combo(s, 4).SelectMany(x => Arrange(x)).ToList();
            list = list.GroupBy(x => new string(x.Substring(0, 2).OrderBy(y => y).ToArray()) + new string(x.Substring(2, 2).OrderBy(y => y).ToArray())).Select(x => x.First()).ToList();
            Console.WriteLine("count: " + list.Count);
            list.ForEach(x => Console.WriteLine(x.Substring(0, 2) + " - " + x.Substring(2, 2)));
        }        static IEnumerable<string> Arrange(string source)
        {
            for (int i = 0; i < source.Length; i++)
            {
                if (source.Length == 1)
                {
                    yield return source;
                }
                else
                {
                    foreach (var x in Arrange(source.Substring(0, i) + source.Substring(i + 1)))
                    {
                        yield return source[i] + x;
                    }
                }
            }
        }        static IEnumerable<string> Combo(string source, int len)
        {
            int[] pos = new int[len];
            for (int i = 0; i < len; i++) pos[i] = i;
            while (pos[0] < source.Length - len)
            {
                string str = "";
                for (int i = 0; i < len; i++) str += source[pos[i]];
                for (int i = len - 1; i >= 0; i--)
                {
                    if (pos[i] < source.Length - len + i)
                    {
                        pos[i]++;
                        for (int j = i + 1; j <= len - 1; j++)
                        {
                            pos[j] = pos[i] + j - i;
                        }
                        break;
                    }
                    else
                    {
                        continue;
                    }
                }
                yield return str;
            }
            yield return source.Substring(source.Length - len);
        }
    }
}