一个面试题 从原数组内生成一个指定大小（长度小于原数组）的数组

[jianjialin]

比如一个int数组长度是100，内容可以是任何数字。现在要在这个数组里面随即取20个项组成新的数组。
请问下面这个代码好吗？还有没有更佳的办法？
谢谢

/// <summary>
        /// 从原数组内生成一个指定大小（长度小于原数组）的数组
        /// </summary>
        /// <param name="alOld">原数组</param>
        /// <param name="count">目标随即数组的大小</param>
        /// <returns>返回新的数组</returns>
        private IList<int> GetQIDs(IList<int> alOld, int length)
        {
            IList<int> newList = new List<int>();
            int min = 0;
            
            while (newList.Count < length)
            {
                int max = alOld.Count-1;
                while (true)
                {
                    Random rnd = new Random();
                    int rand = rnd.Next(min, max + 1);
                    if (!Exists(newList, alOld[rand]))
                    {
                        newList.Add(rand);
                        alOld.RemoveAt(rand);
                        break;
                    }
                }
            }

            return newList;
        }

        private bool Exists(IList<int> al, int n)
        {
            if (al.Contains(n))
                return true;
            else
                return false;
        }

[我姓区不姓区]

        int[] GetQIDs(int[] alOld, int length)
        {
            int[] result = new int[length];
            Random r = new Random(Environment.TickCount);
            int count = 0;
            while (count < length)
            {
                int j = r.Next(0, alOld.Length);
                if (Array.IndexOf(result, alOld[j]) < 0)
                {
                    result[count] = alOld[j];
                    count++;
                }
            }
            return result;
        }

[SealedLove]

/// <summary>
        /// 从原数组内生成一个指定大小（长度小于原数组）的数组
        /// </summary>
        /// <param name="alOld">原数组</param>
        /// <param name="count">目标随即数组的大小</param>
        /// <returns>返回新的数组</returns>
            private IList<int> GetQIDs(IList<int> alOld, int length)
    {
        IList<int> newList = new List<int>();
        int max = alOld.Count;
        if (max >= length)
        {
            while (newList.Count < length)
            {
                Random rnd = new Random();
                int rand = rnd.Next(0, max);
                newList.Add(alOld[rand]);
                alOld.RemoveAt(rand);
                max--;
            }
        }
        return newList;
    }

[阿非]

    /// <summary>
    /// 从原数组内生成一个指定大小（长度小于原数组）的数组
    /// </summary>
    /// <param name="alOld">原数组</param>
    /// <param name="count">目标随即数组的大小</param>
    /// <returns>返回新的数组</returns>
    private IList<int> GetQIDs(IList<int> alOld, int length)
    {
        IList<int> newList = new List<int>();
        
        int min = 0;
        Random rnd = new Random();
        int rand;
        while (newList.Count <= length)
        {                     
            rand = rnd.Next(1, alOld.Count);            
            newList.Add(rand);
            alOld.RemoveAt(rand);                
        }

        return newList;
    }

[zhantianyou]

[Quote=引用 1 楼 ojlovecd 的回复:]
C# codeint[] GetQIDs(int[] alOld,intlength)
{int[] result=newint[length];
Random r=newRandom(Environment.TickCount);intcount=0;while(count<length)
{intj=r.Next(0, alOld.Length);if(Array.IndexOf(result, alOld[j])<0)
{
result[count]=alOld[j];
count++;
}
}returnresult;
}
[/Quote]
這個好.只循環一次.
樓主你那個嵌套了,會影響效率

[阿非]

[Quote=引用 3 楼 Sandy945 的回复:]
C# code
/// <summary>
/// 从原数组内生成一个指定大小（长度小于原数组）的数组
/// </summary>
/// <param name="alOld">原数组</param>
/// <param name="count">目标随即数组的大小</param>
/// <returns>返回新的数组</returns>
private IList<int> GetQIDs(IList<int> alOld, int length)
{
IList<int> newList = new List<int>();

int min = 0;
…
[/Quote]

/// <summary>
    /// 从原数组内生成一个指定大小（长度小于原数组）的数组
    /// </summary>
    /// <param name="alOld">原数组</param>
    /// <param name="count">目标随即数组的大小</param>
    /// <returns>返回新的数组</returns>
    private IList<int> GetQIDs(IList<int> alOld, int length)
    {
        IList<int> newList = new List<int>();
                
        Random rnd = new Random();
        int rand;
        while (newList.Count <= length)
        {                     
            rand = rnd.Next(1, alOld.Count);            
            newList.Add(alOld[rand]);
            alOld.RemoveAt(rand);                
        }

        return newList;
    }