P8649 [蓝桥杯 2017 省 B] k 倍区间 - 洛谷 (luogu.com.cn)

#include <iostream>
#include <vector>
using namespace std;
#define int long long
signed main() {
	int n, k;
	cin >> n >> k;
	vector<int> a(n + 1);
	vector<int> sum(n + 1);
	vector<int> cnt(k);
	sum[0] = 0;
	cnt[0] = 1;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		sum[i] = (sum[i - 1] + a[i]) % k;
		cnt[sum[i]]++;
	}
	int count = 0;
	for (int i = 0; i < k; i++) {
		count += cnt[i] * (cnt[i] - 1) / 2;
	}
	cout << count << endl;
	return 0;
}