【模板】归并排序

 ps：数组下标从0开始哦！

int tmp[MAX],rec[MAX];
int sum;//求逆序对个数 
void merge(int low,int mid,int high)
{
    int i=low,j=mid+1,k=low;
    while(i<=mid&&j<=high)
    {
        if(rec[i]>rec[j]) 
        {
            tmp[k++]=rec[j++];
            sum+=mid-i+1;
        }
        else tmp[k++]=rec[i++];
    }
    while(i<=mid) tmp[k++]=rec[i++];
    while(j<=high) tmp[k++]=rec[j++];
    for(i=low;i<=high;i++) rec[i]=tmp[i];
}
void mergesort(int low,int high) 
{
    if(low<high)
    {
        int mid=(low+high)/2;
        mergesort(low,mid);
        mergesort(mid+1,high);
        merge(low,mid,high);
    }
}