BZOJ2803: [Poi2012]Prefixuffix

#include<map>
#include<set>
#include<deque>
#include<queue>
#include<stack>
#include<cmath>
#include<ctime>
#include<bitset>
#include<string>
#include<vector>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<climits>
#include<complex>
#include<iostream>
#include<algorithm>
#define ll long long
using namespace std;

void read(int &x)
{
char c;
while(!((c=getchar())>='0'&&c<='9'));
x=c-'0';
while((c=getchar())>='0'&&c<='9') x=(x<<3)+(x<<1)+c-'0';
}
void up(int &x,int y){if(x<y)x=y;}
const int maxn = 1100000;
const ll Mod=1e9+7;

char str[maxn];
int f[maxn];
ll h[maxn],hk=233,h2[maxn],hk2=1e8+7;
ll pw[maxn],pw2[maxn];
int n,m;

bool judge(int l,int r,int l2,int r2)
{
ll s1=h[r]-h[l-1]*pw[r-l+1];
ll s2=h[r2]-h[l2-1]*pw[r2-l2+1];
if(s1!=s2) return false;
s1=h2[r]-h2[l-1]*pw2[r-l+1]%Mod;
s2=h2[r2]-h2[l2-1]*pw2[r-l+1]%Mod;
s1=(s1%Mod+Mod)%Mod;
s2=(s2%Mod+Mod)%Mod;
return s1==s2;
}
void g_f()
{
int mi=n>>1; int k=0;
for(int i=mi;i>=1;i--)
{
k+=2; while(i+k-1>mi)k--;
while(k&&!judge(i,i+k-1,n-i+1-k+1,n-i+1))
k--;
f[i]=k;
}
}

int main()
{
read(n);
scanf("%s",str+1);
for(int i=1;i<=n;i++) h[i]=h[i-1]*hk+str[i]-'a';
for(int i=1;i<=n;i++) h2[i]=(h2[i-1]*hk2+str[i]-'a')%Mod;
pw[0]=1ll; for(int i=1;i<=n;i++) pw[i]=pw[i-1]*hk;
pw2[0]=1ll; for(int i=1;i<=n;i++) pw2[i]=pw2[i-1]*hk2%Mod;

g_f();

int L=0; int mi=n>>1;
for(int i=n;i>=n-mi+1;i--)
if(judge(i,n,1,n-i+1)) up(L,(n-i+1)+f[n-i+2]);
printf("%d\n",L);

return 0;
}