CCF模拟题 相反数

#include <bits/stdc++.h>
#define LL long long
using namespace std;
const int maxn = 1010;
int positive[maxn],negative[maxn],n,tmp;
int main(){
while(~scanf("%d",&n)){
LL ans = 0;
memset(positive,0,sizeof(positive));
memset(negative,0,sizeof(negative));
for(int i = 0; i < n; ++i){
scanf("%d",&tmp);
if(tmp < 0) negative[-tmp]++;
if(tmp > 0) positive[tmp]++;
}
for(int i = 0; i < maxn; ++i)
ans += positive[i]*negative[i];
printf("%I64d\n",ans);
}
return 0;
}