Triangle Counting UVA

大白书讲的很好。。

#include <iostream>
#include <cstring>
using namespace std;
typedef long long LL;
const int MAXN = 1001000;
LL n,A[MAXN];
int main()
{

    A[3] = 0;
    for(LL i=4;i<MAXN;i++)
        A[i] = A[i-1] + ((i-1)*(i-2)/2 - (i-1)/2)/2;
    while(cin>>n && n)
    {
        if(n < 3) break;
        cout<<A[n]<<endl;
    }
    return 0;
}