URAL 1297 最长回文子串（LCP模板）

//最长回文子串 , lcp 模板
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef int lint;
const lint maxn = 2000 + 10;
typedef int lint;
struct suffix{

    lint c[maxn],sa[maxn],t1[maxn],t2[maxn],m,n,h[maxn],height[maxn],rk[maxn],s[maxn];

    void Build_SA(lint len){
        m = 0;
        lint* x = t1,*y = t2;
        n = len;
        for( lint i = 0;i < n;i++ ) m = max( m,s[i] );
        for( lint i = 0;i <= m;i++ ) c[i] = 0;
        for( lint i = 0;i < n;i++ ) c[ x[i] = s[i] ]++;
        for( lint i = 1;i <= m;i++ ) c[i] += c[i-1];
        for( lint i = n-1;i >= 0;i-- ) {
            sa[ --c[ x[i] ] ] = i;
        }
        for( lint k = 1;k < n;k <<= 1 ){
            lint cnt = 0;
            for( lint i = n - 1;i >= n-k;i-- ) y[cnt++] = i;
            for( lint i = 0;i < n;i++ ) if( sa[i] >= k ) y[cnt++] = sa[i]-k;
            for( lint i = 0;i <= m;i++ ) c[i] = 0;
            for( lint i = 0;i < n;i++ ) c[ x[i] ]++;
            for( lint i = 1;i <= m;i++ ) c[i] += c[i-1];
            for( lint i = cnt-1;i >= 0;i-- ) sa[ --c[ x[ y[i] ] ] ] = y[i];
            swap( x,y );
            lint num = 0;
            x[ sa[0] ] = 0;
            for( lint i = 1;i < n;i++ ){
                if( y[ sa[i-1] ] != y[ sa[i] ] || y[ sa[i-1]+k ] != y[ sa[i]+k ] ){
                    x[ sa[i] ] = ++num;
                }else{
                    x[ sa[i] ] = num;
                }
            }
            if( num == n-1 ) return;
            m = num;
        }
    }
    void getheight(){
        for( lint i = 0;i < n;i++ ){
            rk[ sa[i] ] = i;
        }
        lint cnt = 1;
        h[ sa[0] ] = 0;
        height[ 0 ] = 0;
        for( lint i = 0;i < n;i++ ){
            if(cnt)cnt--;
            while(  rk[i] >= 1 && i + cnt < n &&sa[rk[i]-1]+cnt < n  && s[ sa[rk[i]-1]+cnt ] == s[ i+cnt ] ) cnt++;
            h[ i ] = cnt;
            height[ rk[i] ] = cnt;
        }
        //cout << "debug" << endl;
    }

}g;
lint rmq[maxn][20],lg[maxn];
void init( lint s,lint t ){
    lint dt = t-s+1;
    lg[1] = 0;
    for( lint i = 2;i <= dt;i++ ) lg[i] = i&(i-1) ? lg[i-1] : lg[i-1] + 1;
}
void build2( lint s,lint t,lint rmq[][20] ){
    lint dt = t-s;
    for( lint i = s;i < t;i++ ){
        rmq[i][0] = g.height[i];
    }
    for( lint i = 1;i <= lg[dt];i++ ){
        for( lint j = s;j + (1 << i) <= t;j++ ){
            rmq[j][i] = min( rmq[j][ i-1 ],rmq[ j + (1 << i-1  ) ][ i-1 ] );
        }
    }
}
lint len;
lint lcp( lint x,lint y,lint rmq[][20] ){
    x = g.rk[x];
    y = g.rk[y];
    x++; y++;
    if( x > y ) swap( x,y );
    if( x >= len || y > len ) return 0;
    lint k = lg[y-x];
    return min( rmq[x][k],rmq[ y - ( 1 << k ) ][k] );
}
char s[maxn],s2[maxn];
int main()
{
    while( ~scanf("%s",s) ){
   lint n =strlen(s);
    strcpy( s2,s );
    reverse( s2,s2+n );
    strcat( s,"#" );
    strcat( s,s2 );
    len = strlen(s);
    for( lint i = 0;i < len;i++ ){
        g.s[i] = s[i] ;
    }
    g.Build_SA(len);
    g.getheight();
    init( 0,len );
    build2( 0,len,rmq );
    lint ans = 1, pos = 0;
    lint re;
    for( lint i = 1;i < n;i++ ){
        if( i < n-1 ){
        re = 2*lcp( len-i,i+1,rmq )+1;
        if( re > ans ){
            ans = re ;
            pos = i - re/2;
        }
        }
        re = 2 *lcp( i,len-i,rmq );
        if( re > ans ){
            ans = re;
            pos = i- re/2;
        }
    }
    for( lint i = pos;i < pos + ans;i++ ){
        printf("%c",s[i]);
    }
    printf("\n");
}
    return 0;
}