題解

此題略神QAQ
orz po神牛

由題我們知道我們要求出：

回文子序列數 - 連續回文子串數

我們記為ans1和ans2

ans2可以用馬拉車輕松解出，這裏就不贅述了

問題是ans1
我們設\(f[i]\)表示以i位置為中心的對稱的字符對數，那麽i位置產生的回文子序列數 = \(2^{f[i]} - 1\)
如何求？
由對稱的性質，以i為對稱中心的兩點\(a,b\)滿足\(a+b=2*i\)
我們可以設一個這樣的序列：
\(c[n]\)表示以\(n/2\)位置為對稱點的對稱點對數【n/2若不為整數則對稱中心是字符間隙】
那麽有：
\(c[n] = \sum a[k]*a[n - k]\)

我們只需要求兩次fft：
①‘a‘位置賦值0，‘b‘位置賦值1，求\(c[n] = \sum a[k]*b[n - k]\)
②‘a‘位置賦值1，‘b‘位置賦值0，求\(c[n] = \sum a[k]*b[n - k]\)

兩次之和即為所求，再跑一次DFT即可【我也不知道為什麽可以這樣，抄po神的代碼】
【講道理分開來求，然後相加應該也行】

最後ans = ans1 - ans2

真心心累。。。

#include<iostream>
#include<cmath>
#include<cstdio>
 

#include<cstring>
#include<algorithm>
#include<complex>
#define LL long long int
#define REP(i,n) for (int i = 1; i <= (n); i++)
#define Redge(u) for (int k = h[u],to; k; k = ed[k].nxt)
#define BUG(s,n) for (int i = 1; i <= (n); i++) cout<<s[i]<<‘ ‘; puts("");
using
 
 namespace std;
const int maxn = 800005,maxm = 200005,INF = 1000000000,P = 1000000007;
inline int read(){
    int out = 0,flag = 1; char c = getchar();
    while (c < 48 || c > 57) {if (c == ‘-‘) flag = -1; c = getchar();}
    while (c >= 48 && c <= 57) {out = (out << 3) + (out << 1) + c - ‘0‘; c = getchar();}
    return out * flag;
}
char s[maxm],t[maxm];
int RL[maxm],n;
LL ans1,ans2,F,power[maxn];
void manacher(){
    s[0] = ‘*‘;
    int pos = 1,mr = 1; RL[1] = 1;
    for (int i = 2; i < n; i++){
        if (i <= mr) RL[i] = min(RL[2 * pos - i],mr - i + 1);
        else RL[i] = 1;
        while (s[i + RL[i]] == s[i - RL[i]]) RL[i]++;
        if (i + RL[i] - 1 >= mr) mr = i + RL[i] - 1,pos = i;
    }
}
const double pi = acos(-1);
typedef complex<double> E;
E a[maxn],b[maxn];
int m,L,R[maxn];
void fft(E* a,int f){
    for (int i = 0; i < n; i++) if (i < R[i]) swap(a[i],a[R[i]]);
    for (int i = 1; i < n; i <<= 1){
        E wn(cos(pi / i),f * sin(pi / i));
        for (int j = 0; j < n; j += (i << 1)){
            E w(1,0);
            for (int k = 0; k < i; k++,w *= wn){
                E x = a[j + k],y = w * a[j + k + i];
                a[j + k] = x + y; a[j + k + i] = x - y;
            }
        }
    }
    if (f == -1) for (int i = 0; i < n; i++) a[i] /= n;
}
int main(){
    scanf("%s",t + 1); int len = strlen(t + 1);
    for (int i = 1; i <= len; i++) s[++n] = ‘#‘,s[++n] = t[i]; s[++n] = ‘#‘;
    manacher();
    for (int i = 1; i <= n; i++) ans2 = (ans2 + (RL[i] >> 1)) % P;
    //cout<<ans2<<endl;
    power[0] = 1; for (int i = 1; i <= n; i++) power[i] = (power[i - 1] << 1) % P;
    n = len;
    m = n << 1; for (n = 1; n <= m; n <<= 1) L++;
    for (int i = 0; i < n; i++) R[i] = (R[i >> 1] >> 1) | ((i & 1) << (L - 1));
    for (int i = 1; i <= len; i++) a[i] = (t[i] == ‘a‘);
    fft(a,1);
    for (int i = 0; i < n; i++) b[i] = a[i] * a[i];
    memset(a,0,sizeof(a));
    for (int i = 1; i <= len; i++) a[i] = (t[i] == ‘b‘);
    fft(a,1);
    for (int i = 0; i < n; i++) b[i] += a[i] * a[i];
    fft(b,-1);
    for (int i = 1; i < n; i++){
        F = (LL)(b[i].real() + 0.5);
        ans1 = (ans1 + power[F + 1 >> 1] - 1) % P;
    }
    //cout<<ans1<<endl;
    printf("%lld\n",((ans1 - ans2) % P + P ) % P);
    return 0;
}

BZOJ3160 萬徑人蹤滅 【fft + manacher】