51nod1305 Pairwise Sum and Divide

 1 #include<cstdio>
 2 #include<algorithm>
 3 using namespace std;
 4 typedef long long ll;
 5 int main(){
 6     int n, i, a;
 7     int s1 = 0, s2 = 0, ss = 0;
 8     scanf("%d", &n);
 9     for(i = 0; i < n; ++i){
10         scanf("%d", &a);
11         if(a == 1) s1++;
12         else if(a == 2) s2++;
13         else  ss++;
14     }
15     ll ans = (ll)s1*(s1-1) + (ll)s1*(s2+ss) + (ll)s2*(s2-1)/2;
16     printf("%lld\n", ans);
17     return 0;
18 }