组合问题

Question

public arraylist a(ArrayList al, int num )
{}
比如这个方法，我传入一个 arrayList 和 num al 里面是一个int 数组
num 表示从al里取几个数字，要求不重复的组合。返回的是 有多少个符合条件的数组

Answer 1

补充： 
比如： 
Al={1，2，3，4，5，6}
num=3就是从 al 中组合
123,124,125,126,134…… 类似这样

Answer 2

public List<int> a(System.Collections.ArrayList al, int num)
        {
            //去掉重复值
            List<int> list = new List<int>();
            for (int i = 0; i < al.Count; i++)
            {
                if(!list.Contains((int)al[i]))
                    list.Add((int)al[i]);
            }            //先进行判断,以减少运算
            if (num > list.Count)
                return list;            List<int> ret = new List<int>();
            for (int i = 0; i < num; i++)
            { 
                Random r = new Random(1);
                int value = r.Next(list.Count);
                if (ret.Contains(list[value]))
                {
                    while (true)
                    {
                        value = r.Next(1, 101);
                        if (!ret.Contains(list[value]))
                        {
                            ret.Add(list[value]);
                            break;
                        }
                    }
                }
                else
                {
                    ret.Add(value);
                }
            }        }

Answer 3

 /// <summary>
        /// 组合
        /// </summary>
        /// <param name="al">int列表集合</param>
        /// <param name="num">组合数</param>
        private IList<string> Combination(ArrayList al, int num)
        {
            IList<string> list = new List<string>();
            IList<string> cbList = new List<string>();
            //删除重复值
            for (int i = 0; i < al.Count; i++)
            {
                if (!list.Contains<string>(al[i].ToString()))
                {
                    list.Add(al[i].ToString());
                }
            }
            if (list.Count < num)
            {
                return null;
            }
            else
            {
                if (num > 1)
                {
                    //递归组合
                    for (int i = 0; i < list.Count; i++)
                    {
                        cbList = cbList.Union<string>(Combination(list[i], list, num)).ToList<string>();
                    }
                }
                else
                {
                    cbList = list;
                }
                return cbList;
            }
        }        /// <summary>
        /// 组合数据
        /// </summary>
        /// <param name="a">要进行组合的数</param>
        /// <param name="list">可与a进行组合的列表</param>
        /// <param name="num">组合长度</param>
        /// <returns>组合结果</returns>
        private IList<string> Combination(string a, IList<string> list, int num)
        {
            IList<string> cbList = new List<string>();
            for (int i = 0; i < list.Count; i++)
            {
                //判断a中是否包含list[i]，若为包含则将其与a组合
                if (!a.Contains(list[i]))
                {
                    cbList.Add(a + list[i]);
                }
            }
            //判断组合后的长度是否等于num，若小于num则继续进行递归组合
            if (cbList.Count > 0 && cbList[0].Length < num)
            {
                IList<string> comList = new List<string>();
                //遍历已组合的集合，继续进行组合
                for (int i = 0; i < cbList.Count; i++)
                {
                    comList = comList.Union<string>(Combination(cbList[i], list, num)).ToList<string>();
                }
                return comList;
            }
            else
            {
                return cbList;
            }
        }