BZOJ2342: [Shoi2011]双倍回文

对每个可能成为本质不同回文子串的串判一下就好了，复杂度$O(n)$。

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#include<cstdio> const int N=5e5+5; int n,s,f[N]; char z[N]; int main(){     scanf("%d ",&n);     fread(z+1,1,n,stdin);     z[0]='^';     for(int i=1,j=0;i<=n;++i){         int k=j+f[j];         if(i<k&&f[j*2-i]<k-i)             f[i]=f[j*2-i];         else{             f[j=i]=i<k?k-i:0;             while(z[i-f[i]]==z[i+f[i]+1]){                 int l=++f[i]/2;                 if(l>s&&f[i-l]>=l)s=l;             }         }     }     printf("%d\n",s*4); }