归并排序及其适用范围

归并排序的运行时间是O(NlogN)，但它很难用于主存排序，主要问题在于合并两个排序的表需要线性附加的内存，而且还需要讲述据拷贝到临时数组仔拷贝回来这样的附加工作，严重地放慢了排序速度。归并排序一般会用在外部排序的算法中。

void Merge(int A[], int TmpArray[], int Lpos, int Rpos, int RightEnd)
{
int i, LeftEnd, NumElements, TmpPos;

LeftEnd = Rpos - 1;
TmpPos = Lpos;
NumElements = RightEnd - Lpos + 1;

while(Lpos <= LeftEnd && Rpos <= RightEnd){
if(A[Lpos] <= A[Rpos])
TmpArray[TmpPos++] = A[Lpos++];
else
TmpArray[TmpPos++] = A[Rpos++];
}

while(Lpos <= LeftEnd)
TmpArray[TmpPos++] = A[Lpos++];
while(Rpos <= RightEnd)
TmpArray[TmpPos++] = A[Rpos++];

for(i = 0; i < NumElements; i++, RightEnd--)
A[RightEnd] = TmpArray[RightEnd];

}

void MSort(int A[], int TmpArray[], int Left, int Right)
{
int Center;

if(Left < Right)
{
Center = (Left + Right)/2;
MSort(A, TmpArray, Left, Center);
MSort(A, TmpArray, Center+1, Right);
Merge(A, TmpArray, Left, Center + 1, Right);

}
}

void Mergesort(int A[], int N)
{
int *TmpArray;

TmpArray = (int *)malloc(N * sizeof(int));
if( TmpArray != NULL)
{
MSort(A, TmpArray, 0, N-1);
free(TmpArray);
}else{
return;
}
}