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| #pragma GCC optimize("2")
#include <bits/stdc++.h>
#define PI 3.14159265
#define PQ priority_queue
#define lowbit(x) ((x) & (-x))
#define rep(i , x , y) for(int (i) = (x) ; (i) <= (y) ; ++i)
#define per(i , x , y) for(int (i) = (x) ; (i) >= (y) ; --i)
#ifdef lmy
#define DEBUG printf("Passing [%s] in LINE %d...\n" , __FUNCTION__ , __LINE__)
#endif
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
inline void read(int &x) {
int f = 1;x = 0;char ch = getchar();
while(ch < '0' || ch > '9') {if(ch == '-') f = -1; ch = getchar();}
while(ch >= '0' && ch <= '9') {x = x * 10 + ch - '0'; ch = getchar();}
x *= f;
}
inline void readll(ll &x) {
ll f = 1;x = 0;char ch = getchar();
while(ch < '0' || ch > '9') {if(ch == '-') f = -1; ch = getchar();}
while(ch >= '0' && ch <= '9') {x = x * 10 + ch - '0'; ch = getchar();}
x *= f;
}
inline void print(int x) {
if (x < 0) putchar('-'), x = -x;if (x > 9) print(x / 10);putchar(x % 10 + 48);
}
inline void printll(ll x) {
if (x < 0) putchar('-'), x = -x;if (x > 9) print(x / 10);putchar(x % 10 + 48);
}
const int MaxN = 2000000 + 5;
const ull bse = 13331;
char a[MaxN];
int n;
ull Pw[MaxN] , Hsh[MaxN];
vector <int> p;
void HshPre() {
Pw[0] = 1;
rep(i , 1 , n + 1) Pw[i] = Pw[i - 1] * bse;
rep(i , 1 , n) {
Hsh[i] = Hsh[i - 1] * bse + (ull)(a[i]);
}
}
ull HshValue(int l , int r) {
return Hsh[r] - Hsh[l - 1] * Pw[r - l + 1];
}
ull Merge(int l , int r , int ql , int qr) {
ull HshValue1 = HshValue(l , r) , HshValue2 = HshValue(ql , qr);
return HshValue1 * Pw[qr - ql + 1] + HshValue2;
}
bool Chksame(int l , int r , int ql , int qr) {
return HshValue(l , r) == HshValue(ql , qr);
}
set <ull> obtain;
void solve() {
cin >> n;
scanf("%s" , (a + 1));
if(!(n % 2)) {
puts("NOT POSSIBLE");
return ;
}
HshPre();
rep(i , 1 , n) {
if(i == 1) {
if(Chksame(2 , (n + 1) / 2 , (n + 1) / 2 + 1 , n) && !obtain . count(HshValue(2 , (n + 1) / 2))) {
p . push_back(i);
obtain . insert(HshValue(2 , (n + 1) / 2));
}
}
else if(i == n) {
if(Chksame(1 , n / 2 , n / 2 + 1 , n - 1) && !obtain . count(HshValue(1 , n / 2))) {
p . push_back(i);
obtain . insert(HshValue(1 , n / 2));
}
}
else if(i == (n + 1) / 2) {
if(Chksame(1 , n / 2 , n - (n / 2) + 1 , n) && !obtain . count(HshValue(1 , n / 2))) {
p . push_back(i) , obtain . insert(HshValue(1 , n / 2));
}
}
else if(i > (n + 1) / 2) {
int l = 1 , r = n / 2;
int pl = r + 1 , pr = i - 1;
int ppl = i + 1 , ppr = n;
ull V = Merge(pl , pr , ppl , ppr);
if(V == HshValue(l , r) && !obtain . count(HshValue(l , r))) {
p . push_back(i);
obtain . insert(HshValue(l , r));
}
}
else if(i < (n + 1) / 2) {
int l = n - (n / 2) + 1 , r = n;
int pl = 1 , pr = i - 1;
int ppl = i + 1 , ppr = l - 1;
ull V = Merge(pl , pr , ppl , ppr);
if(V == HshValue(l , r) && !obtain . count(HshValue(l , r))) {
p . push_back(i);
obtain . insert(HshValue(l , r));
}
}
}
if(p . size() == 0) {
puts("NOT POSSIBLE");
return ;
}
else if(p . size() == 1) {
int _ = p[0];
if(_ == 1) {
rep(i , (n + 1) / 2 + 1 , n) putchar(a[i]);
return ;
}
else if(_ == n) {
rep(i , 1 , n / 2) putchar(a[i]);
return ;
}
else if(_ == (n + 1) / 2) {
rep(i , 1 , n / 2) putchar(a[i]);
return ;
}
else if(_ > (n + 1) / 2) {
rep(i , 1 , n / 2) putchar(a[i]);
return ;
}
else if(_ < (n + 1) / 2) {
rep(i , (n + 1) / 2 + 1 , n) putchar(a[i]);
return ;
}
return ;
}
else if(p . size() > 1) {
puts("NOT UNIQUE");
return ;
}
}
int main(){
#ifdef lmy
int nol_cl = clock();
#endif
solve();
#ifdef lmy
printf("\nTime: %dms\n", int((clock() - nol_cl) / (double)CLOCKS_PER_SEC * 1000));
#endif
return 0;
}
|