ural1297 求最长回文子串 | 后缀数组

#include<cstdio>
#include<algorithm>
#include<cstring>
#define N 20005
using namespace std;
int buf1[N],buf2[N],sa[N],rnk[N],buc[N],n,height[N],ans,Log[N],pos,Min[N][25];
char s[N];
void suffix_sort()
{
    int *x=buf1,*y=buf2,m=1000;
    for (int i=0;i<m;i++) buc[i]=0;
    for (int i=1;i<=n;i++) buc[x[i]=s[i]]++;
    for (int i=1;i<=m;i++) buc[i]+=buc[i-1];
    for (int i=n;i;i--) sa[buc[x[i]]--]=i;
    for (int k=1;k<=n;k<<=1)
    {
        int  p=0;
        for (int i=n-k+1;i<=n;i++)
            y[++p]=i;
        for (int i=1;i<=n;i++)
            if (sa[i]>k) y[++p]=sa[i]-k;
        for (int i=0;i<=m;i++) buc[i]=0;
        for (int i=1;i<=n;i++) buc[x[y[i]]]++;
        for (int i=1;i<=m;i++) buc[i]+=buc[i-1];
        for (int i=n;i;i--) sa[buc[x[y[i]]]--]=y[i];
        swap(x,y),x[sa[1]]=p=1;
        for (int i=2;i<=n;i++)
            if (y[sa[i-1]]==y[sa[i]] && y[sa[i-1]+k]== y[sa[i]+k])
                x[sa[i]]=p;
             else x[sa[i]]=++p;
        if ((m=p)>=n) break;
    }
    for (int i=1;i<=n;i++)
    rnk[sa[i]]=i;
    for (int i=1,j,k=0;i<=n;i++)
    {
        if (rnk[i]==1) continue;
        if (k) k--;
        j=sa[rnk[i]-1];
        while (s[i+k]==s[j+k] && i+k<=n && j+k<=n) k++;
        height[rnk[i]]=k;
    }    
}
void InitRmq()
{
    for (int i=1;i<=n;i++) Min[i][0]=height[i];
    for (int j=1;j<=20;j++)
    for (int i=1;i+(1<<j)-1<=n;i++)
        Min[i][j]=min(Min[i][j-1],Min[i+(1<<j-1)][j-1]);
}
int Query(int x,int y)
{
    int i=rnk[x],j=rnk[y];
    if (i>j) swap(i,j);
    int len=Log[j-i];
    return min(Min[i+1][len],Min[j-(1<<len)+1][len]);
}
int main()
{
    scanf("%s",s+1);
    n=strlen(s+1);
    if (n==1)
    {
    puts(s+1);
    return 0;
    }
    s[n+1]='#';
    for (int i=1;i<=n;i++)
    s[2*n+2-i]=s[i];
    n=n*2+1;
    suffix_sort();
    for (int i=2;i<=n;i++)
    Log[i]=Log[i>>1]+1;
    InitRmq();
    for (int i=1;i<=(n-1)/2;i++)
    {
    int tmp=Query(i,n-i+2);
    if (tmp*2>ans) ans=tmp*2,pos=i-tmp;
    tmp=Query(i,n-i+1);
    if (tmp*2-1>ans) ans=tmp*2-1,pos=i-tmp+1;
    }
    for (int i=1;i<=ans;i++)
    putchar(s[i+pos-1]);
    return 0;
}