【bzoj3160】万径人踪灭 FFT

题目：http://www.lydsy.com/JudgeOnline/problem.php?id=3160

我是一个傻叉 微笑脸

 #include<bits/stdc++.h>
 #define inf 1000000000
 #define ll long long
 #define N 200005
 #define mod 1000000007
 using namespace std;
 int read(){
   int x=,f=;char ch=getchar();
   while(ch<''||ch>''){if(ch=='-')f=-;ch=getchar();}
   while(ch>=''&&ch<=''){x=x*+ch-'';ch=getchar();}
   return x*f;
 }
 struct CD{
   double x,y;
   CD(double a=,double b=){x=a;y=b;}
   friend CD operator + (CD n1,CD n2){return CD(n1.x+n2.x,n1.y+n2.y);}
   friend CD operator - (CD n1,CD n2){return CD(n1.x-n2.x,n1.y-n2.y);}
   friend CD operator * (CD n1,CD n2){return CD(n1.x*n2.x-n1.y*n2.y,n1.x*n2.y+n1.y*n2.x);}
 };
 const double Pi=acos(-1.0);
 int bit,n,nn,ans,m,mx,id,p[N<<];
 char s[N],st[N<<];
 CD a[N<<],b[N<<];
 void FFT(CD *a,int n,int type){
   for(int i=,j=;i<n;i++) {
     if(j>i)swap(a[i],a[j]);
     int k=n;
     while(j&(k >>= ))j&=~k;
     j|=k;
   }
   for(int i=;i<=bit;i++){
     CD w_n(cos(*type*Pi/(<<i)),sin(*type*Pi/(<<i)));
     for(int j=;j<n;j+=(<<i)){
       CD w(,);
       for(int k=j;k<j+(<<(i-));k++){
         CD tmp=a[k],tt=w*a[k+(<<(i-))];
         a[k]=tmp+tt;
         a[k+(<<(i-))]=tmp-tt;
         w=w*w_n;
       }
     }
   }
   if(type<)for(int i=;i<n;i++)a[i].x/=n;
 }
 int main(){
   scanf("%s",s);
   n=strlen(s);nn=n;
   bit=;
   while((<<bit)<(n<<))bit++;
   n=<<bit;
   for(int i=nn;i<n;i++)a[i]=b[i]=CD(,);
   for(int i=;i<nn;i++)if(s[i]=='a')a[i]=CD(,);
   FFT(a,n,);
   for(int i=;i<n;i++)a[i]=a[i]*a[i];
   FFT(a,n,-);
 
   for(int i=;i<nn;i++)if(s[i]=='b')b[i]=CD(,);
   FFT(b,n,);
   for(int i=;i<n;i++)b[i]=b[i]*b[i];
   FFT(b,n,-);
 
   for(int i=;i<n;i++){
     int x=round(a[i].x+b[i].x);
     x=(x+)/;
     ans=(ans+(<<x)-)%mod;
   }
 
   m=nn<<|;
   for(int i=;i<nn;i++)st[(i+)<<]=s[i];
   mx=id=;
   for(int i=;i<=m;i++){
     if(mx>i)p[i]=min(p[*id-i],mx-i);else p[i]=;
     while(i-p[i]->&&i+p[i]+<=m&&st[i-p[i]-]==st[i+p[i]+])p[i]++;
     if(i+p[i]>mx)mx=i+p[i],id=i;
     ans=((ans-(p[i]+)/)%mod+mod)%mod;
   }
   printf("%d\n",ans);
   return ;
 }