BZOJ3916 [Baltic2014]friends

版权声明：蒟蒻Blog随意转载 https://blog.csdn.net/a1799342217/article/details/82024822

哈希

题目传送门

貌似可以直接暴力搞啊。。。

每次枚举一个字符，删掉以后比较s前一半和后一半即可。

代码：

#include<cstdio>
#include<cstring>
#include<algorithm>
#define N 2000005
using namespace std;
typedef unsigned long long ull;
int n,num,a[26]; char s[N];
ull h1,h2,h[N],p[N],ans[N];
inline bool pd(int x){
    if (x<=n>>1){
        h1=h[x-1]*p[(n+1>>1)-x]+h[n+1>>1]-h[x]*p[(n+1>>1)-x];
        h2=h[n]-h[n+1>>1]*p[n-(n+1>>1)];
    }
    else{
        h1=h[n>>1];
        h2=(h[x-1]-h[n>>1]*p[x-1-(n>>1)])*p[n-x]+h[n]-h[x]*p[n-x];
    }
    return h1==h2;
}
inline bool cmp(int x){
    if (!ans[0]){
        ans[0]=h1; int tmp=0;
        for (int i=1;i<=n>>1;i++){
            if (i==x) tmp=1;
            ans[i]=s[i+tmp]-'A';
        }
    }
    return ans[0]==h1;
}
int main(){
    scanf("%d%s",&n,s+1),p[0]=1;
    for (int i=1;i<=n;i++){
        h[i]=h[i-1]*23333+s[i];
        p[i]=p[i-1]*23333;
        a[s[i]-'A']++;
    }
    for (int i=0;i<26;i++) if (a[i]&1) num++;
    if (num!=1) return puts("NOT POSSIBLE"),0;
    for (int i=1;i<=n;i++)
        if (pd(i)&&!cmp(i)) return puts("NOT UNIQUE"),0;
    if (!ans[0]) return puts("NOT POSSIBLE"),0;
    for (int i=1;i<=n>>1;i++) putchar(ans[i]+'A');
    return 0;
}