# | Time | Username | Problem | Language | Result | Execution time | Memory |
---|---|---|---|---|---|---|---|
1249637 | thanh3008 | Cryptography (NOI20_crypto) | C++20 | 19 ms | 7344 KiB |
#include <bits/stdc++.h>
#define TASK ""
#define fi first
#define se second
#define pb push_back
#define ll long long
using namespace std;
const int N=3e5+7,MOD=1e9+7;
ll n,a[N],b[N],giaithua[N],x[N],cnt=0;
bool tangdan=1,giamdan=1,kt[N],ok=0,kdau=1;
void backtrack(int pos){
for(int i=1;i<=n;i++){
if(ok) return;
if(!kt[b[i]]){
x[pos]=b[i];
kt[b[i]]=1;
if(pos>=n){
cnt++;
if(cnt>=MOD) cnt-=MOD;
ok=1;
for(int i=1;i<=n;i++){
if(a[i]!=x[i]){
ok=0;
break;
}
}
if(ok) return;
}
else backtrack(pos+1);
kt[b[i]]=0;
}
}
}
int main(){
if(fopen(TASK".inp","r")){
freopen(TASK".inp","r",stdin);
freopen(TASK".out","w",stdout);
}
ios_base::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
cin>>n;
giaithua[0]=1;
for(int i=1;i<=n;i++){
cin>>a[i];
b[i]=a[i];
giaithua[i]=(giaithua[i-1]*i)%MOD;
}
for(int i=1;i<n;i++){
if(a[i]>a[i+1]){
tangdan=0;
break;
}
}
for(int i=1;i<n;i++){
if(a[i]<a[i+1]){
giamdan=0;
break;
}
}
if(tangdan){
cout<<"1\n";
return 0;
}
else if(giamdan){
cout<<giaithua[n]<<"\n";
return 0;
}
if(a[1]>a[2]);
for(int i=2;i<n;i++){
if(a[i]>a[i+1]){
kdau=0;
break;
}
}
if(kdau){
cout<<((a[1]-1)*giaithua[n-1])%MOD<<"\n";
return 0;
}
if(n<=10){
sort(b+1,b+n+1);
backtrack(1);
cout<<cnt<<"\n";
}
return 0;
}
Compilation message (stderr)
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