[luoguP2863] [USACO06JAN]牛的舞会The Cow Prom（Tarjan）

传送门

有向图，找点数大于1的强连通分量个数

——代码

 #include <stack>
 #include <cstdio>
 #include <cstring>
 #include <iostream>

 const int MAXN = ;
 int n, m, cnt, idx, size, ans;
 int head[MAXN], to[MAXN << ], next[MAXN << ];
 int dfn[MAXN], low[MAXN], belong[MAXN], tot[MAXN];
 bool ins[MAXN];
 std::stack <int> s;

 inline int read()
 {
     int x = , f = ;
     char ch = getchar();
     for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -;
     for(; isdigit(ch); ch = getchar()) x = (x << ) + (x << ) + ch - '';
     return x * f;
 }

 inline int min(int x, int y)
 {
     return x < y ? x : y;
 }

 inline void add(int x, int y)
 {
     to[cnt] = y;
     next[cnt] = head[x];
     head[x] = cnt++;
 }

 inline void dfs(int u)
 {
     int i, v;
     s.push(u);
     ins[u] = ;
     dfn[u] = low[u] = ++idx;
     for(i = head[u]; i ^ -; i = next[i])
     {
         v = to[i];
         if(!dfn[v])
         {
             dfs(v);
             low[u] = min(low[u], low[v]);
         }
         else if(ins[v]) low[u] = min(low[u], dfn[v]);
     }
     if(low[u] == dfn[u])
     {
         size++;
         do
         {
             v = s.top();
             s.pop();
             ins[v] = ;
             belong[v] = size;
         }
         while(u ^ v);
     }
 }

 int main()
 {
     int i, x, y;
     n = read();
     m = read();
     memset(head, -, sizeof(head));
     for(i = ; i <= m; i++)
     {
         x = read();
         y = read();
         add(x, y);
     }
     for(i = ; i <= n; i++)
         if(!dfn[i])
             dfs(i);
     for(i = ; i <= n; i++) tot[belong[i]]++;
     for(i = ; i <= size; i++)
         if(tot[i] > )
             ans++;
     printf("%d\n", ans);
     return ;
 }