。。恩 打了四五遍 不会也背出来了。。
BZOJ3160 【听说时限紧？转C++的优势么？】
上AC代码 fft
1 /*Problem: 3160 2 User: cyz666 3 Language: C++ 4 Result: Accepted 5 Time:1992 ms 6 Memory:18492 kb 7 ****************************************************************/ 8 9 #include <bits/stdc++.h> 10 #define LL long long 11 const LL mo=1000000007; 12 const double pi=acos(-1.0); 13 using namespace std; 14 struct cp{ 15 double x,y; 16 cp(double a=0,double b=0):x(a),y(b){} 17 //cp(double a=0,double b=0){x=a,y=b;} 18 }a[400005],b[400005],W[200005]; 19 int h[400005],rev[400005],f[200005],c[200005],n,x,m; 20 char s; LL ans; 21 cp operator +(cp a,cp b){ 22 cp c(a.x+b.x,a.y+b.y); 23 return c; 24 } 25 cp operator *(cp a,cp b){ 26 cp c(a.x*b.x-a.y*b.y,a.x*b.y+a.y*b.x); 27 return c; 28 } 29 cp operator -(cp a,cp b){ 30 return (cp){a.x-b.x,a.y-b.y}; 31 } 32 void fft(cp a[],int n,int d=1){ 33 if (d<0) reverse(a+1,a+n); 34 for (int i=0;i<n;++i) 35 if (i<rev[i]) swap(a[i],a[rev[i]]); 36 for (int i=0;i<n/2;++i) 37 W[i]=cp(cos(2*pi/n*i),sin(2*pi/n*i)); 38 for (int m=2,k=1;m<=n;k=m,m<<=1) 39 for (int i=0;i<n;i+=m) 40 for (int j=i;j<i+k;++j){ 41 cp u=a[j],v=a[j+k]*W[n/m*(j-i)]; 42 a[j]=u+v,a[j+k]=u-v; 43 } 44 if (d<0) for (int i=0;i<n;++i) a[i].x/=n; 45 } 46 int main(){ 47 while (scanf("%c",&s)&&isalpha(s)) 48 s=='a'?c[++++n]=1:c[++++n]=2; 49 c[0]=-1; c[n+2]=-2; 50 for (int i=1;i<=n;++i){ 51 if (x+f[x]>i) f[i]=min(f[x+x-i],x+f[x]-i); 52 while (c[i+f[i]]==c[i-f[i]]) ++f[i]; --f[i]; 53 if (i+f[i]>x+f[x]) x=i; 54 ans+=f[i]/2; if (ans>=mo) ans-=mo; 55 } 56 m=ceil((log(n)/log(2))); m=1<<m; 57 for (int i=1;i<m;++i)rev[i]=(rev[i>>1]>>1)+(m>>1)*(i&1); 58 x=0; for (int i=2;i<=n;++++i) c[++x]=c[i]; n=x; 59 for (int i=1;i<=n;++i) c[i]==1?a[i].x=1:b[i].x=1; 60 fft(a,m); fft(b,m); 61 for (int i=0;i<m;++i) a[i]=a[i]*a[i],b[i]=b[i]*b[i]; 62 fft(a,m,-1); fft(b,m,-1); 63 h[0]=1; for (int i=1;i<=n;++i)h[i]=(h[i-1]+h[i-1])%mo; 64 int x; ans=mo-ans; 65 for (int i=1;i<n+n;++i){ 66 x=round(a[i].x+b[i].x); 67 ans+=h[x+1>>1]-1; if (ans>=mo) ans-=mo; 68 } 69 ans=(ans-n+1+mo)%mo; 70 printf("%lld",ans); 71 return 0; 72 }