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：

分解：将问题分级为若干个子问题

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

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}\t", 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}\t", 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++;
}
}
}