ural1297

题解：

后缀数组

st表处理加速ｌｃｐ

把串后面加一个不可能出现的字符

然后再把串倒过来放在后面

暴力枚举中心

判断ｌｃｐ

代码：

#include<bits/stdc++.h>
using namespace std;
const int N=;
int ws1[N],wv[N],wa[N],wb[N],rank1[N],height[N],sa[N],a[N],n,dp[N][];
char str[N];
int cmp(int *r,int a,int b,int l)
{
    return r[a]==r[b]&&r[a+l]==r[b+l];
}
void da(int *r,int *sa,int n,int m)
{
    int *x=wa,*y=wb;
    for (int i=;i<m;i++)ws1[i]=;
    for (int i=;i<n;i++)ws1[x[i]=r[i]]++;
    for (int i=;i<m;i++)ws1[i]+=ws1[i-];
    for (int i=n-;i>=;i--)sa[--ws1[x[i]]]=i;
    for (int j=,p=;p<n;j*=,m=p)
     {
         p=;
        for (int i=n-j;i<n;i++)y[p++]=i;
        for (int i=;i<n;i++)
         if (sa[i]>=j)y[p++]=sa[i]-j;
        for (int i=;i<n;i++)wv[i]=x[y[i]];
        for (int i=;i<m;i++)ws1[i]=;
        for (int i=;i<n;i++)ws1[wv[i]]++;
        for (int i=;i<m;i++)ws1[i]+=ws1[i-];
        for (int i=n-;i>=;i--)sa[--ws1[wv[i]]]=y[i];
        p=;swap(x,y);x[sa[]]=;
        for (int i=;i<n;i++)x[sa[i]]=cmp(y,sa[i-],sa[i],j)?p-:p++;
    }
}
void calheight(int *r,int *sa,int n)
{
    int j,k=;
    for (int i=;i<=n;i++)rank1[sa[i]]=i;
    for (int i=;i<n;height[rank1[i++]]=k)
     for (k?k--:,j=sa[rank1[i]-];r[i+k]==r[j+k];k++);
}
void RMQ()
{
    memset(dp,,sizeof(dp));
    for (int i=;i<=n*+;i++)dp[i][]=height[i];
    for (int j=;(<<j)<=*n+;j++)
     for (int i=;i+(<<j)-<=*n+;i++)
      dp[i][j]=min(dp[i][j-],dp[i+(<<(j-))][j-]);
}
int lcp(int l,int r)
{
    int a=rank1[l],b=rank1[r];
    if (a>b)swap(a,b);
    a++;
    int t=(int)(log(double(b-a+))/log(2.00));
    return min(dp[a][t],dp[b-(<<t)+][t]);
}
int main()
{
    int res,flag,max;
    while (~scanf("%s",str))
     {
        max=;
        n=strlen(str);
        for (int i=;i<n;i++)a[i]=(int)str[i];
        a[n]=;
        for (int i=;i<n;i++)a[i+n+]=int(str[n-i-]);
        a[*n+]=;
        da(a,sa,n*+,);
        calheight(a,sa,*n+);
        RMQ();
        for (int i=;i<n;i++)
         {
            res=lcp(i,*n-i)*-;
            if (max<res)
             {
                max=res;
                flag=i;
             }
            if (i>)
             {
                res=lcp(i,*n-i+)*;
                if (max<res)
                 {
                    max=res;
                    flag=i;
                 }
             }
         }
        if (max%==)
         for (int i=flag-max/;i<=flag+max/;i++)printf("%c",str[i]);
        else
         for (int i=flag-max/;i<=flag+max/-;i++)printf("%c",str[i]);
        puts("");
     }
    return ;
}