分治算法

结构上是递归的，为了解决给定的问题，算法一次或者多次递归调用自身，解决紧密相关的子问题，通常有三个步骤：

1. 分解：将问题分级为若干个子问题
2. 解决：对子问题进行递归求解，如果子问题足够小，则直接求解
3. 合并：子问题的解还原为原问题的解

使用归并法排序：

static void Main(string[] args)
{
    //产生随机序列
    var rand = new Random();
    List<int> sequence = new List<int>();
    int length = 18;
    for (int i = 0; i < length; i++)
    {
        sequence.Add(rand.Next()%10);
        Console.Write("{0}	", sequence[i]);
    }
    //归并法排序
    Merge_Sort(sequence, 0, length-1);
    //输出排序后的序列
    Console.WriteLine();
    for (int i = 0; i < length; i++)
    {
        Console.Write("{0}	", sequence[i]);
    }
}
static void Merge_Sort(List<int> sq, int s, int e)
{
    if (s < e)
    {
        int q = (s + e) / 2;
        Merge_Sort(sq, s, q);
        Merge_Sort(sq, q+1, e);
        Merge(sq, s, q, e);
    }
}
static void Merge(List<int> sq, int s, int m, int e)
{
    List<int> sequenceL = new List<int>();
    for (int i = s; i <= m; i++)
        sequenceL.Add(sq[i]);
    List<int> sequenceR = new List<int>();
    for (int i = m+1; i <= e; i++)
        sequenceR.Add(sq[i]);
    int li = 0, ri = 0;
    for (int k = s; k <= e; k++)
    {
        if (li >= sequenceL.Count)
        {
            sq[k] = sequenceR[ri];
            ri++;
            continue;
        }
        if (ri >= sequenceR.Count)
        {
            sq[k] = sequenceL[li];
            li++;
            continue;
        }
        if (sequenceL[li] < sequenceR[ri])
        {
            sq[k] = sequenceL[li];
            li++;
        }
        else
        {
            sq[k] = sequenceR[ri];
            ri++;
        }
    }
}