Topologic Sorting Algorithm 我好像悟了！

#include <iostream>
#include <vector>
#include <queue>

using namespace std;

int n, m, u, v;

int main(void) {
  scanf("%d %d", &n, &m);

  // build the directed graph
  vector<vector<int>> g(n + 1);
  vector<int> indegrees(n + 1); // 入度表
  vector<int> paths(n + 1);

  while (m--) {
    scanf("%d %d", &u, &v);
    ++indegrees[v];
    g[u].emplace_back(v);
  }

  queue<int> q;
  for (int i = 1; i <= n; ++i)
    if (!indegrees[i]) paths[i] = 1, q.emplace(i);

  int ans = 0;
  while (not q.empty()) {
    const int cur = q.front(); q.pop();
    for (const auto& nei : g[cur]) {
      paths[nei] += paths[cur];
      if (--indegrees[nei] == 0) {
        q.emplace(nei);
        if (g[nei].empty()) ans += paths[nei];
      }
    }
  }

  printf("%d\n", ans);
}