3817: Sum

题目链接

题目大意：给定正整数N，R；求$\sum\limits_{d=1}^{n}(-1)^{\lfloor d \times r \times d \rfloor }$

题解：膜CA

我的收获：经典套路

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

int T;
ll n,r;
double t;

inline ll calc(ll n,ll a,ll b,ll c){
    if(!n) return 0;
    ll g=__gcd(__gcd(a,b),c);a/=g,b/=g,c/=g;
    ll k=(t*b+c)/a,ret=n*(n+1)/2*k;
    c-=k*a;k=(t*b+c)/a*n;
    return ret+n*k-calc(k,b*b*r-c*c,a*b,-a*c);
}

void work()
{
    scanf("%lld%lld",&n,&r);t=sqrt(r);
    if((ll)t==t) printf("%d\n",(r&1)?((n&1)?-1:0):n);
    else printf("%lld\n",n+(calc(n,2,1,0)<<2)-(calc(n,1,1,0)<<1));
}

int main()
{
    scanf("%d",&T);
    while(T--) work();
    return 0;
}