Language: C++

#include<bits/stdc++.h>
#define sqr(x) ((x)*(x))
#define ll long long
#define ull unsigned long long
#define ui unsigned int
#define ld long double
#define vi vector<int>
#define HEAP(...) priority_queue<__VA_ARGS__ >
#define heap(...) priority_queue<__VA_ARGS__,vector<__VA_ARGS__ >,greater<__VA_ARGS__ > > 
#define pii pair<int,int> 
#define pb push_back
#define mp make_pair
#define debuge cerr<<"isok"<<endl
#define debug(x) cerr<<#x<<"="<<x<<endl
#define dprintf(...) fprintf(stderr,__VA_ARGS__)
#define SS second
#define FF first
#define ls (k<<1)
#define rs (k<<1|1)
#define clr(a,x) memset(a,x,sizeof(a))
#define cpy(a,x) memcpy(a,x,sizeof(a))
#define file(x) freopen(x".in","r",stdin),freopen(x".out","w",stdout)
#define SZ(x) ((int)x.size())
using namespace std;
template<class T> inline void gmin(T &x,const T &y){if(x>y) x=y;}
template<class T> inline void gmax(T &x,const T &y){if(x<y) x=y;}
const int BufferSize=1<<16;
char buffer[BufferSize],*Bufferhead,*Buffertail;
bool Terminal;
inline char Getchar(){
	if(Bufferhead==Buffertail){
		int l=fread(buffer,1,BufferSize,stdin);
		if(!l){Terminal=1;return 0;}
		Buffertail=(Bufferhead=buffer)+l;
	}
	return *Bufferhead++;
}
template<class T>inline bool read(T &x){
	x=0;char c=Getchar(),rev=0;
	while(c<'0'||c>'9'){c=Getchar();rev|=c=='-';if(Terminal)return 0;}
	while(c>='0'&&c<='9') x=x*10+c-'0',c=Getchar();
	if(c=='.'){
		c=Getchar();double t=0.1;
		while(c>='0'&&c<='9') x=x+(c-'0')*t,c=Getchar(),t=t/10;
	}
	x=rev?-x:x;
	return 1;
}
template<class T1,class T2> inline bool read(T1 &x,T2 &y){return read(x)&read(y);}
template<class T1,class T2,class T3> inline bool read(T1 &x,T2 &y,T3 &z){return read(x)&read(y)&read(z);}
template<class T1,class T2,class T3,class T4> inline bool read(T1 &x,T2 &y,T3 &z,T4 &w){return read(x)&read(y)&read(z)&read(w);}
inline bool reads(char *x){
	char c=Getchar();
	while(c<33||c>126){c=Getchar();if(Terminal)return 0;}
	while(c>=33&&c<=126) (*x++)=c,c=Getchar();
	*x=0;return 1;
}
template<class T>inline void print(T x,const char c='\n'){
	if(!x){putchar('0');putchar(c);return;}
	if(x<0) putchar('-'),x=-x;
	int m=0,a[20];
	while(x) a[m++]=x%10,x/=10;
	while(m--) putchar(a[m]+'0');
	putchar(c);
}
//--------------------------------head---------------------------------------------

const int inf=0x3f3f3f3f;
const int N=100005,M=100005,mod=1e9+7;
template<class T,class S> inline void ch(T &x,const S y){x=(x+y)%mod;}
inline int exp(int x,int y,const int mod=::mod){
	int ans=1;
	while(y){
		if(y&1) ans=(ll)ans*x%mod;
		x=(ll)x*x%mod;y>>=1;
	}return ans;
}

int fac[N],ifac[N],pw[N];
int n,res,cnt,a[N],du[N],si[N],s[N];
bool vis[N],v[N];
vi g[N];

inline int C(int x,int y){
	if(x<y) return 0;
	return (ll)fac[x]*ifac[y]%mod*ifac[x-y]%mod;
}
void dfs(int x){
	vis[x]=1;cnt++;
	if(du[x]==2){
		int l=g[x][0],r=g[x][1],p=2;
		vi L(0),R(0);L.pb(l);R.pb(r);
		while(du[l]&&du[r]){
			if(du[l]+du[r]>2){puts("0");exit(0);}
			l=g[l][0],r=g[r][0],p+=2,L.pb(l),R.pb(r);
		}
		if(du[l]) swap(l,r);
		for(int j:L) vis[j]=1;
		for(int j:R) vis[j]=1;
		for(int i=0,j=cnt+2;i<L.size();i++,j+=2)
			s[L[i]]=-j;
		for(int i=0,j=cnt+2;i<R.size();i++,j+=2)
			s[R[i]]=-j;
		if(du[r]==2){
			for(int j=cnt+1;j<=cnt+p;j++)
				v[j]=1;
			for(int i=0,j=cnt+1;i<L.size();i++,j+=2)
				s[L[i]]=j;
			for(int i=0,j=cnt+2;i<R.size();i++,j+=2)
				s[R[i]]=j;
			cnt+=p-1;dfs(r);
		}
		else if(du[r]==1){
			res++;cnt+=p;
			v[cnt+1]=1;s[g[r][0]]=cnt+1;
			dfs(g[r][0]);
		}
		else cnt+=p,res++;
	}
	else if(du[x]==1){
//		debug(x);
		v[cnt+1]=1;s[g[x][0]]=cnt+1;
		dfs(g[x][0]);
	}
}
void dfs2(int x){
	vis[x]=1;cnt++;
	for(int y:g[x])
		if(!vis[y]) dfs2(y);
}

int main(){
#ifdef rqgao2014
	freopen("input.txt","r",stdin);
#endif
	read(n);
	pw[0]=1;
	for(int i=1;i<=n;i++) pw[i]=pw[i-1]*2%mod;
	fac[0]=1;
	for(int i=1;i<=n;i++) fac[i]=(ll)fac[i-1]*i%mod;
	ifac[n]=exp(fac[n],mod-2);
	for(int i=n;i;i--) ifac[i-1]=(ll)ifac[i]*i%mod;
	for(int i=1;i<=n;i++){
		read(a[i]);
		g[a[i]].pb(i);du[a[i]]++;
	}
	for(int i=1;i<=n;i++)
		if(du[i]>2){puts("0");return 0;}
	int ans=1;
	for(int i=1;i<=n;i++)
		if(!vis[i]&&du[i]>1){
			debug(i);
			vi tmp=g[a[i]];g[a[i]].clear();
			for(int j:tmp) if(j!=i) g[a[i]].pb(j);du[a[i]]--;
			cnt=res=0;
			v[1]=1;s[i]=1;
			dfs(i);
			if(abs(s[a[i]])<cnt-1){puts("0");return 0;}
			if(s[a[i]]<0&&(v[cnt-1]||v[cnt])) res--;
			ans=(ll)ans*pw[res]%mod;
			for(int j=1;j<=cnt;j++) v[j]=0;
		}
	for(int i=1;i<=n;i++)
		if(!vis[i]){
			cnt=0;dfs2(i);
			si[cnt]++;
		}
//	debug(ans);
	for(int i=1;i<=n;i++){
		res=0;
		gmax(mx,si[i]);
		for(int j=0,now=1;j<=si[i]/2;j++){
			if(i%2==0||i==1) ch(res,now); else ch(res,(ll)now*pw[si[i]-j*2]);
			now=(ll)now*C(si[i]-j*2,2)%mod*i%mod*exp(j+1,mod-2)%mod;
		}
		ans=(ll)ans*res%mod;
	}
	print(ans);
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:171:22: error: 'mx' was not declared in this scope; did you mean 'mp'?
  171 |                 gmax(mx,si[i]);
      |                      ^~
      |                      mp