1257: [CQOI2007]余数之和

题目链接

bzoj1257: [CQOI2007]余数之和

题解

数论分块，乘等差数列求和

代码

#include<bits/stdc++.h>
using namespace std;
inline int read() {
    int x = 0,f = 1;
    char c = getchar();
    while(c < '0' || c > '9')c = getchar();
    while(c <= '9' && c >= '0')x = x * 10 + c - '0',c = getchar();
    return x * f;
}
long long n ,k ;
int main() {
    n = read(),k = read();
    long long ans = n * k;
    for(long long l = 1,r;l <= n;l = r + 1 ) {
            if(k / l) r = min(n,k / (k / l));
            else r =  n;
            ans -=  (r - l + 1) * (k / l) * (l + r) >> 1;
    }
    printf("%lld\n",ans);
    return 0;
}