【BZOJ2190】【SDOI2008】仪仗队

【题目链接】

• 点击打开链接
  

【思路要点】

• 补档博客，无题解。

【代码】

#include<bits/stdc++.h>
using namespace std;
#define MAXN	50005
template <typename T> void read(T &x) {
	x = 0; int f = 1;
	char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;
	for (; isdigit(c); c = getchar()) x = x * 10 + c - '0';
	x *= f;
}
int tot, prime[MAXN];
int f[MAXN], miu[MAXN];
void init() {
	miu[1] = 1;
	for (int i = 2; i < MAXN; i++) {
		if (f[i] == 0) {
			f[i] = prime[++tot] = i;
			miu[i] = -1;
		}
		for (int j = 1; j <= tot && prime[j] <= f[i]; j++) {
			int tmp = prime[j] * i;
			if (tmp >= MAXN) break;
			f[tmp] = prime[j];
			if (prime[j] == f[i]) miu[tmp] = 0;
			else miu[tmp] = -miu[i];
		}
	}
}
int main() {
	init();
	int n, ans = 0; read(n);
	n--;
	for (int i = 1; i <= n; i++)
		ans += miu[i] * (n / i) * (n / i);
	cout << ans + 2 << endl;
	return 0;
}