代码：

#include <iostream>
#include <vector>
#include <stack>

using namespace std;


int dfs(vector<vector<int>>& cities, int k, vector<int>& temp)
{
    int count = 0;
    stack<int> stack;
    vector<bool> 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<vector<int>> cities(vertexNum + 1, vector<int>());
    for (int i = 0; i < edgesNum; i++)
    {
        int u, v;
        cin >> u >> v;
        cities[u].push_back(v);
    }

    vector<int> s_in(vertexNum + 1, 0);//记录能进入每个城市的总数量
    vector<int> 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;
}