1 //#pragma comment(linker, "/STACK:102400000, 102400000") 2 #include <algorithm> 3 #include <iostream> 4 #include <cstdlib> 5 #include <cstring> 6 #include <cstdio> 7 #include <vector> 8 #include <cmath> 9 #include <ctime>10 #include <list>11 #include <set>12 #include <map>13 using namespace std;14 typedef __int64 LL;15 typedef pair <int, int> P;16 const int N = 2e5 + 5;17 struct Edge {18     int next, to;   19 }edge[N << 1];20 LL mod = 1e9 + 7, d[N], ans, f[N], sum;21 int head[N], cnt;22 bool vis[N];23 24 LL fpow(LL a, LL b) {25     LL res = 1;26     while(b) {27         if(b & 1)28             res = res * a % mod;29         a = a * a % mod;30         b >>= 1;31     }32     return res;33 }34 35 inline void add(int u, int v) {36     edge[cnt].next = head[u];37     edge[cnt].to = v;38     head[u] = cnt++;39 }40 41 void dfs(int u, int p, int dep) {42     if(!ans && vis[u]) {43         ans = dep - d[u] ? dep - d[u] : 2;44         return ;45     } else if(vis[u]) {46         return ;47     }48     d[u] = dep;49     vis[u] = true;50     ++sum;51     for(int i = head[u]; ~i; i = edge[i].next) {52         int v = edge[i].to;53         if(v == p)54             continue;55         dfs(v, u, dep + 1);56     }57 }58 59 int main()60 {61     f[0] = 1;62     for(int i = 1; i < N; ++i) {63         f[i] = f[i - 1]*2LL % mod; //2的阶乘预处理64     }65     memset(head, -1, sizeof(head));66     cnt = 0;67     int n, u;68     scanf("%d", &n);69     for(int i = 1; i <= n; ++i) {70         scanf("%d", &u);71         add(u, i);72         add(i, u);73     }74     LL res = 1;75     for(int i = 1; i <= n; ++i) {76         if(!vis[i]) {77             ans = sum = 0;78             dfs(i, -1, 1);79             res = ((res * (f[ans] - 2) % mod + mod) % mod * f[sum - ans]) % mod;80         }81     }82     printf("%lld\n", res);83     return 0;84 }