SCC template using tarjan

scc.cpp

#include <bits/stdc++.h>
#define ll long long
#define FASTIO   \
  cin.tie(NULL); \
  ios::sync_with_stdio(false);
#define END return 0;
#define out cout <<
#define in cin >>
using namespace std;

struct DirectedGraph {
  int n;
  vector<vector<int>> adj;
  vector<int> dfn, low, scc_id;
  vector<bool> in_stk;
  stack<int> stk;
  int dfs_cnt = 0, scc_cnt = 0;
  vector<vector<int>> sccs;
  DirectedGraph(int n) : n(n), adj(n+1), dfn(n+1), low(n+1), scc_id(n+1), in_stk(n+1) {}
  void add_edge(int u, int v) { adj[u].push_back(v); }
  void tarjan(int u) {
    dfn[u] = low[u] = ++dfs_cnt;
    stk.push(u);
    in_stk[u] = true;
    for (int v : adj[u]) {
      if (!dfn[v]) {
        tarjan(v);
        low[u] = min(low[u], low[v]);
      } else if (in_stk[v]) {
        low[u] = min(low[u], dfn[v]);
      }
    }
    if (dfn[u] == low[u]) {
      vector<int> temp;
      while (true) {
        int v = stk.top();
        stk.pop();
        in_stk[v] = false;
        scc_id[v] = scc_cnt;
        temp.push_back(v);
        if (v == u) break;
      }
      sort(temp.begin(), temp.end());
      sccs.push_back(temp);
      scc_cnt++;
    }
  }
  void build_scc() {
    for (int i = 1; i <= n; i++)
      if (!dfn[i]) tarjan(i);
  }
};

int main() {
  FASTIO
  int V, E;in V>>E;
  DirectedGraph g(V);
  while(E--) {
      int A, B;
      in A>>B;
      g.add_edge(A, B);
  }
  g.build_scc();
  sort(g.sccs.begin(), g.sccs.end(),[](const vector<int>& a, const vector<int>& b) { return a[0] < b[0]; });
  out g.sccs.size()<<"\n";
  for (vector<int>& temp : g.sccs) {
    for (int v : temp) out v<<" ";
    out -1<<"\n";
  }
  END
}