POJ_2299 Ultra-QuickSort【归并排序】

#include <iostream>
#include <cstdio>
#include <cstring>
#define N 500001
using namespace std;
int a
;
int temp
;
long long ans;
void merge(int *a,int start,int mid,int end,int *temp)
{
int ai=start;
int bi=mid+1;
int k=0;
while (ai<=mid&&bi<=end)
{
if(a[ai]<=a[bi])
temp[k++]=a[ai++];
else
{
temp[k++]=a[bi++];
ans+=(mid-ai+1);
}
}
while (ai<=mid)
temp[k++]=a[ai++];
while (bi<=end)
temp[k++]=a[bi++];
for(int i=0;i<=end-start;i++)
{
a[start+i]=temp[i];
}
}
void merge_array(int *a,int start,int end)
{
if(start<end)
{
int mid=(start+end)/2;
merge_array(a,start,mid);
merge_array(a,mid+1,end);
merge(a,start,mid,end,temp);
}
}
int main()
{
int n;
scanf("%d",&n);
while(n)
{
memset(a,0,sizeof(a));
ans=0;
for(int i=0;i<n;i++)
scanf("%d",&a[i]);
merge_array(a,0,n-1);
printf("%I64d\n",ans);
scanf("%d",&n);
}
return 0;
}