注意manacher的写法 注释掉的是TLE的代码
1 #include<bits/stdc++.h>
2 #define ll long long
3 #define db double
4 using namespace std;
5 const int N=150010;
6 const ll mod=1e9+7;
7 const db PI=acos(-1);
8 struct Complex{
9 db real,image;
10 Complex(db _real=0,db _image=0):real(_real),image(_image){}
11 Complex operator+(Complex k)const{return Complex(real+k.real,image+k.image);}
12 Complex operator-(Complex k)const{return Complex(real-k.real,image-k.image);}
13 Complex operator*(Complex k)const{return Complex(real*k.real-image*k.image,real*k.image+image*k.real);}
14 Complex operator/(int k)const{return Complex(real/k,image/k);}
15 }a[N<<1],b[N<<1];
16 int n,m,l,maxp,f[N<<1],rev[N<<1],pow2[N<<1];
17 char s[N];
18 void fft(Complex* x,int f){//f=1时为fft，f=-1时为ifft
19 for(int i=0;i<n;i++) if(rev[i]>i) swap(x[i],x[rev[i]]);
20 for(int len=1;len<n;len<<=1){
21 Complex wn(cos(PI/len),f*sin(PI/len));
22 for(int now=0;now<n-1;now+=(len<<1)){
23 Complex w(1,0);
24 for(int i=0;i<len;i++,w=w*wn){
25 Complex t1=x[now+i],t2=w*x[now+len+i];
26 x[now+i]=t1+t2,x[now+len+i]=t1-t2;
27 }
28 }
29 }
30 if(f==-1) for(int i=0;i<n;i++) x[i]=x[i]/n;
31 return;
32 }
33 int maxr,pos,r[N<<1];
34 //int manacher(){
35 // int ans=0;
36 // s2[l2++]='#';
37 // for(int i=0;i<l;i++) s2[l2++]=s[i],s2[l2++]='#';
38 // for(int i=0;i<l2;i++){
39 // r[i]=(maxr>=i)?min(maxr-i+1,r[(pos<<1)-i]):1;
40 // while(i-r[i]>=0&&i+r[i]<l2&&s2[i-r[i]]==s2[i+r[i]]) r[i]++;
41 // if(r[i]>maxr) maxr=r[i],pos=i;
42 // ans=ans+(r[i]>>1);
43 // if(ans>mod) ans-=mod;
44 // }
45 // return ans;
46 //}
47 int manacher(){
48 int ans=0;
49 s[l+1]='#',s[0]='$';
50 for(int i=1;i<=l;i++){
51 if(maxr>i) r[i]=min(maxr-i+1,r[pos*2-i]);
52 else r[i]=1;
53 while(s[i+r[i]]==s[i-r[i]]) r[i]++;
54 if(i+r[i]-1>maxr) maxr=i+r[i]-1,pos=i;
55 ans=(ans+r[i])%mod;
56 }
57 maxr=0;
58 for(int i=1;i<=l;i++){
59 if(maxr>i) r[i]=min(maxr-i,r[pos*2-i]);
60 else r[i]=0;
61 while(s[i+r[i]+1]==s[i-r[i]]) r[i]++;
62 if(i+r[i]>maxr) maxr=i+r[i],pos=i;
63 ans=(ans+r[i])%mod;
64 }
65 return ans;
66 }
67 int main(){
68 scanf("%s",s+1);
69 l=strlen(s+1),n=l-1;
70 m=n<<1;
71 for(n=1;n<=m;n<<=1) maxp++;
72 for(int i=1;i<n;i++) rev[i]=(rev[i>>1]>>1)|((i&1)<<(maxp-1));
73 for(int i=1;i<=l;i++) a[i-1].real=(s[i]=='a'),b[i-1].real=(s[i]=='b');
74 fft(a,1);fft(b,1);
75 for(int i=0;i<n;i++) a[i]=a[i]*a[i],b[i]=b[i]*b[i];
76 fft(a,-1);fft(b,-1);
77 pow2[0]=1,f[0]=int(a[0].real+b[0].real+1.1)>>1;
78 for(int i=1;i<n;i++){
79 pow2[i]=pow2[i-1]<<1,f[i]=int(a[i].real+b[i].real+1.1)>>1;
80 if(pow2[i]>=mod) pow2[i]-=mod;
81 }
82 ll ans=0;
83 for(int i=0;i<n;i++) ans=(ans+pow2[f[i]]-1)%mod;
84 int temp=manacher();
85 ans-=temp;
86 if(ans<0) ans+=mod;
87 printf("%lld",ans);
88 return 0;
89 }
