[P2671][NOIP2015]求和 (数论)

[题目链接]

这位大神讲得很详细：点我

本蒟蒻只会抄抄题解了

#include<bits/stdc++.h>
#define max(a,b) (a>b?a:b)
#define min(a,b) (a<b?a:b)
#define ll long long
#define N 100005
#define mod 10007
#define inc(a,b) (a=(a+b)%mod)
using namespace std;
inline int read() {
    int f = 1, x = 0; char ch;
    do { ch = getchar(); if (ch == '-')f = -1; } while (ch<'0' || ch>'9');
    do { x = x * 10 + ch - '0'; ch = getchar(); } while (ch >= '0'&&ch <= '9');
    return f * x;
}
int a[N],c[N],f[2][N],d[2][N];
int ans;
int main()
{
    #ifdef onlinejudge
        freopen("sum.in","r",stdin);
        freopen("sum.out","w",stdout);
    #endif
    int n=read(),m=read();
    for(int i=1;i<=n;i++) a[i]=read()%mod;
    for(int i=1;i<=n;i++)
    {
        c[i]=read();
        inc(f[i%2][c[i]],a[i]);
        inc(d[i%2][c[i]],1);
    }
    for(int i=1;i<=n;i++)
        inc(ans,(i%mod * ((f[i%2][c[i]]+(d[i%2][c[i]]-2)%mod*a[i]+mod)%mod)));
    cout<<ans;
}