bzoj 3916 friends | 字符串hash

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<cmath>

#define md
#define ull unsigned long long
#define inf (int) 1e9
#define eps 1e-8
#define N 2000010
using namespace std;
ull mi
,f
,cg=27,hash;
char st
;
int ans=0;
void solve(int x,ull ha)
{
	if (ans==0||hash==ha) ans=x,hash=ha; else { printf("NOT UNIQUE\n"); exit(0); }
}
inline ull get(int l,int r)
{
	ull a=f[r]-f[l-1]*mi[r-l+1];
	return a;
}
	
int main()
{
	int n;
	scanf("%d",&n);
	if (!(n&1)) { printf("NOT POSSIBLE\n"); return 0;}
	mi[0]=1; for (int i=1;i<=n;i++) mi[i]=mi[i-1]*cg;
	scanf("%s",st+1);
	for (int i=1;i<=n;i++) f[i]=f[i-1]*cg+(ull)(st[i]-'A'+1);
	int mid=n>>1;
	ull a,b;
	for (int i=1;i<=mid;i++)
	{
		a=get(1,i-1)*mi[mid-i+1]+get(i+1,mid+1);
		b=get(mid+2,n);
		if (a==b) solve(i,a);
	}
	a=f[mid]; b=f
-f[mid+1]*mi[mid];
	if (a==b) solve(mid+1,a);
	for (int i=mid+2;i<=n;i++)
	{
		a=f[mid];
		b=get(mid+1,i-1)*mi[n-i]+get(i+1,n);
		if (a==b) solve(i,a);
	}
	if (!ans) { printf("NOT POSSIBLE\n"); return 0;}
	if (ans<=mid+1) for (int i=mid+2;i<=n;i++) printf("%c",st[i]);
	  else for (int i=1;i<=mid;i++) printf("%c",st[i]);
	printf("\n");
	return 0;
}