UVA 11401 Triangle Counting——计数原理

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const int maxn = 1e6 + 10;
long long a[maxn];

int main() {
    a[3] = 0;
    for (long long x = 4; x <= maxn; x++) {
        a[x] = a[x-1] + ((x-1)*(x-2)/2 - (x-1)/2)/2;
    }
    int n;
    while (~scanf("%d", &n)) {
        if (n < 3) break;
        cout << a[n] << endl;
    }
    return 0;
}