uva 1363 - Joseph's Problem(数论)

题目链接：uva 1363 - Joseph's Problem

题目大意：给定n，k，求∑i=1n(k%i).

解题思路：参考别人的，自己想了很久，详细题解

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>

using namespace std;
typedef long long ll;

ll solve (ll n, ll k) {
    ll sum = 0;

    if (n > k)
        sum += (n-k) * k;

    ll a = sqrt(k+0.5), b = k / a;
    for (ll i = a; i > 1; i--) {
        ll s = k / i;
        ll e = k / (i-1);

        if (s > n)
            break;

        if (e > n)
            e = n;

        sum += ( (k%(s+1) + k%e) * (e - s) / 2);
    }

    for (ll i = 2; i <= b && i <= n; i++)
        sum += k%i;
    return sum;
}

int main () {
    ll n, k;
    while (scanf("%lld%lld", &n, &k) == 2) {
        printf("%lld\n", solve(n, k));
    }
    return 0;
}