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代码：

#include 
   
    #include 
    
     #include 
     
      using namespace std;int dfs(vector
      
       
        >& cities, int k, vector
        
         & temp){ int count = 0; stack
         
           stack; vector
          
            visited(cities.size(), false); stack.push(k); while (!stack.empty()) { int curNode = stack.top(); if (visited[curNode]) { stack.pop(); } else { visited[curNode] = true; for (int i = 0; i < cities[curNode].size(); i++) { int v = cities[curNode][i]; if (!visited[v]) stack.push(v); } } } for (int i = 1; i< cities.size(); i++) { if (visited[i]) { temp[i] = temp[i] + 1;//如果城市k能访问到城市i，则城市i的进入路线城市加1 ++count;//记录能进入城市i的所有城市总和 } } return count;}int main(){ int vertexNum = 0; int edgesNum = 0; cin >> vertexNum >> edgesNum; vector
           
            
             > cities(vertexNum + 1, vector
             
              ()); for (int i = 0; i < edgesNum; i++) { int u, v; cin >> u >> v; cities[u].push_back(v); } vector
              
                s_in(vertexNum + 1, 0);//记录能进入每个城市的总数量 vector
               
                 s_out(vertexNum + 1, 0);//记录每个城市到达其他城市的总数量 for (int i = 1; i <= vertexNum; i++) { s_out[i] = dfs(cities, i, s_in); } int importCityNum = 0; for (int i = 1; i <= vertexNum; i++) { if (s_in[i] > s_out[i]) importCityNum++; } cout << importCityNum << endl;}