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#include<ctime>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<map>
#include<set>
#include<queue>
#include<deque>
#include<stack>
#include<bitset>
#include<vector>
#include<algorithm>
#include<iostream>
#include<deque>
using namespace std;
namespace mine
{
typedef long long ll;
#define double long double
const int INF=0x3f3f3f3f;
int ()
{
int ans=0;char c=getchar();int f=1;
while(c<'0' or c>'9') {if(c=='-') f=-1;c=getchar();}
while('0'<=c and c<='9') ans=ans*10+c-'0',c=getchar();
return ans*f;
}
void write(int num)
{
if(num<0) {num=-num;putchar('-');}
if(num>9) write(num/10);
putchar('0'+num%10);
}
void writeln(int num){write(num);puts("");}
#define FR first
#define SE second
#define MP make_pair
#define pr pair<int,int>
#define PB push_back
inline void chmax(int &x,int y) {x=x>y?x:y;}
inline void chmin(int &x,int y) {x=x<y?x:y;}
const int MAX_N=1e5+10;
int n;char str[MAX_N];
struct SA
{
int sa[MAX_N],rk[MAX_N],tmp[MAX_N],ct[MAX_N],wr[MAX_N*2];
void getsa()
{
memset(ct,0,sizeof ct);
for(int i=1;i<=n;i++) ct[rk[i]=str[i]]++;
for(int i=1;i<MAX_N;i++) ct[i]+=ct[i-1];
for(int i=n;i>=1;i--) sa[ct[rk[i]]--]=i;
int ln=1;
while(ln<n)
{
int tot=0;for(int i=1;i<=n;i++) if(sa[i]+ln>n) tmp[++tot]=sa[i];
for(int i=1;i<=n;i++) if(sa[i]-ln>=1) tmp[++tot]=sa[i]-ln;
memset(ct,0,sizeof ct);memset(wr,0,sizeof wr);
for(int i=1;i<=n;i++) ct[ wr[tmp[i]]=rk[tmp[i]] ]++;
for(int i=1;i<MAX_N;i++) ct[i]+=ct[i-1];
for(int i=n;i>=1;i--) sa[ct[ wr[tmp[i]] ]--]=tmp[i];
int cnt=1;rk[sa[1]]=1;
for(int i=2;i<=n;i++)
{
if(wr[sa[i]]!=wr[sa[i-1]] or wr[sa[i]+ln]!=wr[sa[i-1]+ln]) cnt++;
rk[sa[i]]=cnt;
}
ln*=2;
}
}
int hei[MAX_N];
void gethei()
{
int lst=0;
for(int i=1;i<=n;i++)
{
if(rk[i]==1) continue;
int j=sa[rk[i]-1];if(lst) lst--;
while(max(i,j)+lst<=n and str[i+lst]==str[j+lst]) lst++;
hei[rk[i]]=lst;
}
}
}sa;
struct STB
{
int bin[30],lg[MAX_N],mm[MAX_N][20];
void pre()
{
bin[0]=1;for(int i=1;i<30;i++) bin[i]=bin[i-1]<<1;
lg[1]=0;for(int i=2;i<MAX_N;i++) lg[i]=lg[i>>1]+1;
for(int i=1;i<20;i++) for(int j=2;j<=n-bin[i]+1;j++) mm[j][i]=min(mm[j][i-1],mm[j+bin[i-1]][i-1]);
}
int ask(int x,int y)
{
int t=lg[y-x+1];
return min(mm[x][t],mm[y-bin[t]+1][t]);
}
}stb;
int K;int mk[MAX_N];
bool check(int pos,int len)
{
memset(mk,63,sizeof mk);mk[pos]=pos+len-1;mk[n]=n;
for(int i=1;i<=n;i++) if(sa.rk[i]>sa.rk[pos])
{
int lcp=stb.ask(sa.rk[pos]+1,sa.rk[i]);chmin(lcp,len);
if(lcp==0) return 0;
mk[i]=i+lcp-1;
}
int mi=n+1,cnt=0;
for(int i=1;i<=n;i++)
{
chmin(mi,mk[i]);
if(mi==i) cnt++,mi=n+1;
}
return cnt<=K;
}
void main()
{
while(1)
{
scanf("%d",&K);if(K==0) break;
scanf("%s",str+1);n=strlen(str+1);
sa.getsa();sa.gethei();
for(int i=2;i<=n;i++) stb.mm[i][0]=sa.hei[i];
stb.pre();
int l=1,r=n,pos;
while(l<=r)
{
int mid=(l+r)/2;
if(check(sa.sa[mid],n-sa.sa[mid]+1)) pos=sa.sa[mid],r=mid-1;
else l=mid+1;
}
l=1,r=n-pos+1;int len;
while(l<=r)
{
int mid=(l+r)/2;
if(check(pos,mid)) len=mid,r=mid-1;
else l=mid+1;
}
for(int i=1;i<=len;i++) putchar(str[pos+i-1]);
puts("");
}
}
};
int main()
{
srand(time(0));
mine::main();
}
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