tried all mentioned test cases .all correct. still WA on #12.
using surfix array.any idea?

code here:

#include <cstdio>
#include <cstring>
int const N=220000;
int st[256], rank[2*N], rank1[2*N], count[N], tmp[N];
char a[N], b[N], s[N], max1;
int sa[N], height[N], si;
int main(){
    memset(st, 0, sizeof st);
    memset(rank, 0, sizeof rank);
    memset(rank1, 0, sizeof rank1);
    scanf("%s", b);
    int n1=strlen(b);
    for(int i=1; i<=n1; ++i)a[i]=b[i-1];
    for(int i=1; i<=n1; ++i)a[i+n1+1]=b[n1-i];
    a[0]=' ';
    a[n1+1]='#';
    int n=2*n1+1;
    a[n+1]='&';
    for(int i=1; i<=n; ++i)st[a[i]]=1;
    for(int i=1; i<=255; ++i)st[i]+=st[i-1];
    for(int i=1; i<=n; ++i)rank[i]=st[a[i]];

    int k=0;
    for(int p=1; k!=n; p+=p){
        memset(count, 0, sizeof count);
        for(int i=1; i<=n; ++i)count[rank[i+p]]++;
        for(int i=1; i<=n; ++i)count[i]+=count[i-1];
        for(int i=n; i>=1; --i)tmp[count[rank[i+p]]--]=i;

        memset(count, 0, sizeof count);
        for(int i=1; i<=n; ++i)count[rank[i]]++;
        for(int i=1; i<=n; ++i)count[i]+=count[i-1];
        for(int i=n; i>=1; --i)sa[count[rank[tmp[i]]]--]=tmp[i];

        memcpy(rank1, rank, sizeof rank1);
        rank[sa[1]]=k=1;
        for(int i=2; i<=n; ++i){
            if(rank1[sa[i]]!=rank1[sa[i-1]] or rank1[sa[i]+p]!=rank1[sa[i-1]+p])++k;
            rank[sa[i]]=k;
        }
    }
    k=0;
    for(int i=1; i<=n; ++i){
        if(rank[i]==1){
            k=0;
        }else{
            --k;
            if(k<0)k=0;
            while(a[i+k]==a[sa[rank[i]-1]+k])++k;
        }
        height[rank[i]]=k;
    }

    int max=-1;
    for(int i=2; i<=n; ++i){
        int p=sa[i];
        int q=sa[i-1];
        if((p-1)/n1 != (q-1)/n1){
            if(p+q+height[i]==n+2){
                if(height[i]>max){
                    max=height[i];
                    si=p;
                }
            }
        }
    }
    for(int i=si; i<si+max; ++i)printf("%c", a[i]);
    return 0;
}

THX :)