ural 1297 O(nlogn) 后缀数组求最长回文字串

#include<iostream>
#include<cstring>
#include<set>
#include<map>
#include<cmath>
#include<stack>
#include<queue>
#include<deque>
#include<list>
#include<algorithm>
#include<stdio.h>
#include<iomanip>

#define rep(i,n) for(int i=0;i<n;++i)
#define fab(i,a,b) for(int i=a;i<=b;++i)
#define fba(i,b,a) for(int i=b;i>=a;--i)
#define PB push_back
#define INF 0x3f3f3f3f
#define MP make_pair
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define sf scanf
#define pf printf
#define LL long long
const int N=2105;
const int M=2105;
using namespace std;
typedef pair<int,int>PII;
int wa[N],wb[N],wv[N],wd[M];
int sa[N],rank[N],height[N];
int LEN;
int d[N][32];
char str[N];
int cmp(int *r, int a, int b, int l){  
    return r[a] == r[b] && r[a+l] == r[b+l];  
}  
//r数组的长度n从[0,n)
//求出的sa为从[1,n]
//调用时候r[n-1]=0
void da(char *r, int n, int m){          //  倍增算法 r为待匹配数组  n为总长度 m为字符范围  
    int i, j, p, *x = wa, *y = wb, *t;  
    for(i = 0; i < m; i ++) wd[i] = 0;  
    for(i = 0; i < n; i ++) wd[x[i]=r[i]] ++;  
    for(i = 1; i < m; i ++) wd[i] += wd[i-1];  
    for(i = n-1; i >= 0; i --) sa[-- wd[x[i]]] = i;  
    for(j = 1, p = 1; p < n; j *= 2, m = p){  
        for(p = 0, i = n-j; i < n; i ++) y[p ++] = i;  
        for(i = 0; i < n; i ++) if(sa[i] >= j) y[p ++] = sa[i] - j;  
        for(i = 0; i < n; i ++) wv[i] = x[y[i]];  
        for(i = 0; i < m; i ++) wd[i] = 0;  
        for(i = 0; i < n; i ++) wd[wv[i]] ++;  
        for(i = 1; i < m; i ++) wd[i] += wd[i-1];  
        for(i = n-1; i >= 0; i --) sa[-- wd[wv[i]]] = y[i];  
        for(t = x, x = y, y = t, p = 1, x[sa[0]] = 0, i = 1; i < n; i ++){  
            x[sa[i]] = cmp(y, sa[i-1], sa[i], j) ? p - 1: p ++;  
        }  
    }  
}  
// 求出的 height [1,n] 对应sa中的[1,n]
// 求出的 rank [0,n-1] 对应sa中的[1,n]
// 调用时候最后添加的0不用加进去
void calHeight(char *r, int n){           //  求height数组。  
    int i, j, k = 0;  
    for(i = 1; i <= n; i ++) rank[sa[i]] = i;  
    for(i = 0; i < n; height[rank[i++]] = k){  
        for(k ? k -- : 0, j = sa[rank[i]-1]; r[i+k] == r[j+k]; k ++);  
    }  
}

//为了统一，所有的下标从0开始,左闭右开！
void fix(int n){
    rep(i,n)sa[i]=sa[i+1],rank[i]--,height[i]=height[i+1];
}
void rmq_init(int n){
   rep(i,n)d[i][0]=height[i];
   for(int j=1;(1<<j)<=n;j++){
       for(int i=0;i+(1<<j)-1<n;i++)
           d[i][j]=min(d[i][j-1],d[i+(1<<(j-1))][j-1]);
   }
} 
int qurry(int L,int R){
    if(L>R)swap(L,R);
    L++;
    int k=0;
    while((1<<(k+1))<=R-L+1)k++;
    return min(d[L][k],d[R-(1<<k)+1][k]);
}
void solve(){
   LEN=strlen(str);
   str[LEN]=1;
   rep(i,LEN)str[LEN+i+1]=str[LEN-i-1];
   str[2*LEN+1]=0;
   da(str,2*LEN+2,300);
   calHeight(str,2*LEN+1);
   fix(2*LEN+1);
   rmq_init(2*LEN+2);
   int n=2*LEN+1;
   int ans=1,pos=0;
   rep(i,LEN){
       int tmp=qurry(rank[i],rank[n-i-1]);
       int len=2*tmp-1;
       if(len>ans){
           ans=len;
           pos=i-tmp+1;
       }
       if(!i)continue;
       tmp=qurry(rank[i],rank[n-i]);
       len=2*tmp;
       if(len>ans){
           ans=len;
           pos=i-tmp;
       }
   }
   rep(i,ans)putchar(str[i+pos]);puts("");
}
int main(){
    while(~sf("%s",str)){
        solve();
    }
    return 0;
}