Tarjan UVALive 6511 Term Project

题目传送门

 /*
     题意：第i个人选择第a[i]个人，问组成强联通分量(自己连自己也算)外还有多少零散的人
     有向图强联通分量-Tarjan算法：在模板上加一个num数组，记录每个连通分量的点数，若超过1，则将连通点数相加
         用总点数-ans则是零散的点
     详细解释：http://www.bubuko.com/infodetail-927304.html
 */
 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #include <vector>
 #include <stack>
 using namespace std;

 const int MAXN = 1e5 + ;
 const int INF = 0x3f3f3f3f;
 vector<int> G[MAXN];
 stack<int> S;
 int pre[MAXN], low[MAXN];
 int instack[MAXN], num[MAXN];
 int dfs_clock, scc_cnt;
 int n, ans;

 void DFS(int u)
 {
     instack[u] = ;
     pre[u] = low[u] = ++dfs_clock;
     S.push (u);
     for (int i=; i<G[u].size (); ++i)
     {
         int v = G[u][i];
         if (!pre[v])
         {
             DFS (v);    low[u] = min (low[u], low[v]);
         }
         else if (instack[v] == )
         {
             low[u] = min (low[u], pre[v]);
         }
     }

     if (low[u] == pre[u])
     {
         scc_cnt++;
         for (; ; )
         {
             int x = S.top ();    S.pop ();
             instack[x] = ;
             num[scc_cnt]++;
             if (x == u)    break;
         }
     }
 }

 void Tarjan(void)
 {
     dfs_clock = scc_cnt = ;
     memset (pre, , sizeof (pre));
     memset (low, , sizeof (low));
     memset (num, , sizeof (num));
     memset (instack, , sizeof (instack));
     while (!S.empty ())    S.pop ();

     for (int i=; i<=n; ++i)
     {
         if (!pre[i])    DFS (i);
     }
 }

 int main(void)        //UVALive 6511 Term Project
 {
     freopen ("L.in", "r", stdin);

     int t;    scanf ("%d", &t);
     while (t--)
     {
         ans = ;    scanf ("%d", &n);
         for (int i=; i<=n; ++i)    G[i].clear ();
         for (int i=; i<=n; ++i)
         {
             int x;    scanf ("%d", &x);
             G[i].push_back (x);
             if (i == x)    ans++;
         }

         Tarjan ();
         for (int i=; i<=scc_cnt; ++i)
         {
             if (num[i] > )    ans += num[i];
         }
         printf ("%d\n", n - ans);
     }

     return ;
 }