[CF711D]Directed Roads（强联通分量，计数）

  1 #include <algorithm>
  2 #include <iostream>
  3 #include <iomanip>
  4 #include <cstring>
  5 #include <climits>
  6 #include <complex>
  7 #include <fstream>
  8 #include <cassert>
  9 #include <cstdio>
 10 #include <bitset>
 11 #include <vector>
 12 #include <deque>
 13 #include <queue>
 14 #include <stack>
 15 #include <ctime>
 16 #include <set>
 17 #include <map>
 18 #include <cmath>
 19 using namespace std;
 20 #define fr first
 21 #define sc second
 22 #define cl clear
 23 #define BUG puts("here!!!")
 24 #define W(a) while(a--)
 25 #define pb(a) push_back(a)
 26 #define Rint(a) scanf("%d", &a)
 27 #define Rll(a) scanf("%I64d", &a)
 28 #define Rs(a) scanf("%s", a)
 29 #define Cin(a) cin >> a
 30 #define FRead() freopen("in", "r", stdin)
 31 #define FWrite() freopen("out", "w", stdout)
 32 #define Rep(i, len) for(int i = 0; i < (len); i++)
 33 #define For(i, a, len) for(int i = (a); i < (len); i++)
 34 #define Cls(a) memset((a), 0, sizeof(a))
 35 #define Clr(a, x) memset((a), (x), sizeof(a))
 36 #define Full(a) memset((a), 0x7f7f7f, sizeof(a))
 37 #define lrt rt << 1
 38 #define rrt rt << 1 | 1
 39 #define pi 3.14159265359
 40 #define RT return
 41 #define lowbit(x) x & (-x)
 42 #define onecnt(x) __builtin_popcount(x)
 43 typedef long long LL;
 44 typedef long double LD;
 45 typedef unsigned long long ULL;
 46 typedef pair<int, int> pii;
 47 typedef pair<string, int> psi;
 48 typedef pair<LL, LL> pll;
 49 typedef map<string, int> msi;
 50 typedef vector<int> vi;
 51 typedef vector<LL> vl;
 52 typedef vector<vl> vvl;
 53 typedef vector<bool> vb;
 54 
 55 const int maxn = 200200;
 56 const LL mod = 1e9+7;
 57 LL ret;
 58 
 59 typedef struct Edge {
 60     int u;
 61     int v;
 62     int next;
 63     Edge() { next = -1; }
 64 }Edge;
 65 
 66 int head[maxn], ecnt;
 67 Edge edge[maxn];
 68 int n, m;
 69 
 70 int bcnt, dindex;
 71 int dfn[maxn], low[maxn];
 72 int stk[maxn], top;
 73 int belong[maxn];
 74 int in[maxn], out[maxn];
 75 bool instk[maxn];
 76 int cnt[maxn];
 77 
 78 void init() {
 79     memset(edge, 0, sizeof(edge));
 80     memset(head, -1, sizeof(head));
 81     memset(instk, 0, sizeof(instk));
 82     memset(dfn, 0, sizeof(dfn));
 83     memset(low, 0, sizeof(low));
 84     memset(belong, 0, sizeof(belong));
 85     memset(in, 0, sizeof(in));
 86     memset(out, 0, sizeof(out));
 87     ecnt = top = bcnt = dindex = 0;
 88 }
 89 
 90 void adde(int uu, int vv) {
 91     edge[ecnt].u = uu;
 92     edge[ecnt].v = vv;
 93     edge[ecnt].next = head[uu];
 94     head[uu] = ecnt++;
 95 }
 96 
 97 void tarjan(int u) {
 98     int v = u;
 99     dfn[u] = low[u] = ++dindex;
100     stk[++top] = u;
101     instk[u] = 1;
102     for(int i = head[u]; ~i; i=edge[i].next) {
103         v = edge[i].v;
104         if(!dfn[v]) {
105             tarjan(v);
106             low[u] = min(low[u], low[v]);
107         }
108         else if(instk[v]) low[u] = min(low[u], dfn[v]);
109     }
110     if(dfn[u] == low[u]) {
111         bcnt++;
112         do {
113             v = stk[top--];
114             instk[v] = 0;
115             belong[v] = bcnt;
116             cnt[bcnt]++;
117         } while(v != u);
118     }
119 }
120 
121 LL quickmul(LL x, LL n) {
122     LL ans = 1;
123     while(n) {
124         if(n & 1) ans = (ans * x) % mod;
125         x = x * x % mod;
126         n >>= 1;
127     }
128     return ans;
129 }
130 
131 
132 int main() {
133     // FRead();
134     while(~Rint(n)) {
135         ret = 1; Cls(cnt);
136         init();
137         int v;
138         For(i, 1, n+1) {
139             Rint(v);
140             adde(i, v);
141         }
142         For(i, 1, n+1) if(!dfn[i]) tarjan(i);
143         LL tmp = 0;
144         For(i, 1, bcnt+1) {
145             if(cnt[i] > 1) {
146                 ret = (ret * (quickmul(2, cnt[i])-2+mod))%mod;
147                 tmp = (tmp + cnt[i]) % mod;
148             }
149         }
150         ret = (ret * quickmul(2, n-tmp)) % mod;
151         cout << ret << endl;
152     }
153     RT 0;
154 }