bzoj4827 [hnoi2017]礼物

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 const int mod=998244353;
 5 const int rt=31;
 6 const int N=500006;
 7 int l,_n,w[N],pos[N],n,k,wn,inv_n;
 8 ll sum1,sum2,ans,a[N],b[N];
 9 void up(int &x,int y) {x=((ll)x+y)%mod;}
10 int ksm(int x,int y)
11 {
12     int res=1;
13     while (y) {if (y&1) res=(ll)res*x%mod; x=(ll)x*x%mod; y>>=1;} 
14     return res; 
15 }
16 void init(int n)
17 {
18     l=0;
19     while ((1<<l)<=n) l++;
20     _n=1<<l;wn=ksm(rt,1<<23-l);
21     w[0]=1;inv_n=ksm(_n,mod-2);
22     for (int i=1;i<_n;i++)
23       w[i]=(ll)w[i-1]*wn%mod,pos[i]=(i&1)?pos[i-1]|(1<<(l-1)):pos[i>>1]>>1;
24 }
25 void fft(ll *a,int op)
26 {
27     for (int i=0;i<_n;i++) if (i<pos[i]) swap(a[i],a[pos[i]]);
28     int len=1,id=_n;
29     for (int i=0;i<l;i++)
30     {
31       int wn=w[id>>=1];
32       for (int j=0;j<_n;j+=len*2)
33         for (int k=j,w=1;k<j+len;k++)
34         {
35              int l=a[k],r=(ll)a[k+len]*w%mod;
36              a[k]=((ll)l+r)%mod;a[k+len]=((ll)l-r+mod)%mod;
37              w=(ll)w*wn%mod;
38          }
39       len<<=1;
40     }
41     if (op==-1) {
42         reverse(a+1,a+_n);
43         for (int i=0;i<_n;i++) a[i]=(ll)a[i]*inv_n%mod; 
44     }
45 }
46 
47 int main()
48 {
49     scanf("%d%d",&n,&k);init(2*n+1);
50     for (int i=1;i<=n;i++) scanf("%lld",&a[i]),sum1+=a[i]*a[i],sum2+=a[i];
51     for (int i=1;i<=n;i++) scanf("%lld",&b[n-i+1]),sum1+=b[n-i+1]*b[n-i+1],sum2-=b[n-i+1];
52      fft(a,1);fft(b,1);
53     for (int i=0;i<_n;i++) a[i]=(ll)a[i]*b[i]%mod;
54     fft(a,-1);
55     for (int i=2;i<=n+1;i++) ans=max(ans,(ll)a[i]+a[n+i]);//取max的时候别取模 
56     double c=round((double)-sum2/n);//注意要加double！ 
57      ll cc=c;
58     ans=(ll)cc*cc*n+2*sum2*cc+sum1-2*ans;
59     printf("%lld\n",ans);
60     return 0;
61 }