poj2299归并排序

http://poj.org/problem?id=2299

#include<stdio.h>
int a[500005];
long long count;

void merge(int first,int mid,int last)
{
    int s1,s2,f1,f2,i,k,f[500005];
    s1=first;
    s2=mid+1;
    f1=mid;
    f2=last;
    k=0;
    while(s1<=f1&&s2<=f2)
    {
        if(a[s1]<a[s2])
        f[k++]=a[s1++];
        else
        {
            f[k++]=a[s2++];
            count+=mid-s1+1;
        }
    }
    while(s2<=f2)
    f[k++]=a[s2++];
    while(s1<=f1)
    f[k++]=a[s1++];
    for(i=first;i<=last;i++)
    a[i]=f[i-first];
}

void mergesort(int start,int end)
{
    int mid=0;
    if(start==end)
    return;
    mid=(start+end)/2;
    mergesort(start,mid);
    mergesort(mid+1,end);
    merge(start,mid,end);
}

int main()
{
    int n,i;
    while(scanf("%d",&n)&&n)
    {
        for(i=0;i<n;i++)
        scanf("%d",&a[i]);
        count = 0;
        mergesort(0,n-1);
        printf("%lld\n",count);
    }
    return 0;
}

用时3672MS。有大神用时141MS。求更优题解！