Submission #548523

#	Time	Username	Problem	Language	Result	Execution time	Memory
548523		krit3379	Three Friends (BOI14_friends)	C++17	100 / 100	173 ms	36496 KiB

friends

#include<bits/stdc++.h>
using namespace std;
#define N 2000005

long long hx[N],po[N],p=31,cnt,ans,l,r,mod=1e9+7;
char s[N];
map<long long,int> mp;

int main(){
    int n,i,mid;
    scanf("%d %s",&n,s+1);
    po[0]=1;
    for(i=1;i<=n;i++)po[i]=po[i-1]*p%mod;
    for(i=1;i<=n;i++)hx[i]=(hx[i-1]*p+s[i]-'A'+1)%mod;
    mid=(n+1)/2;
    if(hx[mid-1]==(hx[n]-hx[mid]*po[n-mid]%mod+mod)%mod)mp[hx[mid-1]],ans=mid;
    if((hx[mid]-hx[1]*po[mid-1]%mod+mod)%mod==(hx[n]-hx[mid]*po[n-mid]%mod+mod)%mod)mp[(hx[mid]-hx[1]*po[mid-1]%mod+mod)%mod],ans=1;
    if(hx[mid-1]==(hx[n-1]-hx[mid-1]*po[n-mid]%mod+mod)%mod)mp[hx[mid-1]],ans=n;
    for(i=2;i<mid;i++){
        l=(hx[i-1]*po[mid-i]%mod+hx[mid]-hx[i]*po[mid-i]%mod+mod)%mod;
        r=(hx[n]-hx[mid]*po[n-mid]%mod+mod)%mod;
        if(l==r)mp[l],ans=i;
    }
    for(i=mid+1;i<n;i++){
        l=hx[mid-1];
        r=((hx[i-1]-hx[mid-1]*po[i-mid]%mod+mod)%mod*po[n-i]%mod+(hx[n]-hx[i]*po[n-i]%mod+mod)%mod)%mod;
        if(l==r)mp[l],ans=i;
    }
    if(mp.size()==0)printf("NOT POSSIBLE");
    else if(mp.size()>1)printf("NOT UNIQUE");
    else{
        for(i=1,cnt=0;cnt<mid-1;i++)if(i!=ans)printf("%c",s[i]),cnt++;
    }
    return 0;
}

Compilation message (stderr)

friends.cpp: In function 'int main()':
friends.cpp:11:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   11 |     scanf("%d %s",&n,s+1);
      |     ~~~~~^~~~~~~~~~~~~~~~

• Subtask 1: 35 / 35
• Subtask 2: 65 / 65