advance_graph_algorithms/Flight_Routes_Check.cpp at main · androemeda/advance_graph_algorithms · GitHub

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#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <string>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <cassert>
#include <limits>
#include <numeric>
#include <climits>
#define int long long
using namespace std;

int n, m;
vector<vector<int>> adj;
vector<vector<int>> revadj;
vector<int> leaders;
vector<int> order;
vector<bool> visited;

void findorder(int node)
{
    visited[node] = true;
    for (int nb : adj[node])
    {
        if (!visited[nb])
        {
            findorder(nb);
        }
    }
    order.push_back(node);
}

void dfs(int node, int leader)
{
    visited[node] = true;
    leaders[node] = leader;
    // cout << node << " ";
    for (int nb : revadj[node])
    {
        if (!visited[nb])
        {
            dfs(nb, leader);
        }
    }
}

void solve()
{
    cin >> n >> m;

    adj.assign(n + 1, vector<int>());
    revadj.assign(n + 1, vector<int>());
    leaders.assign(n + 1, -1);
    visited.assign(n + 1, false);

    for (int i = 0; i < m; i++)
    {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        revadj[v].push_back(u);
    }

    for (int i = 1; i <= n; i++)
    {
        if (!visited[i])
        {
            findorder(i); //using adj here.
        }
    }

    // for(int i=0 ; i<order.size() ; i++){
    //     cout << order[i] << " ";
    // }
    // cout << endl;

    for (int i = 1; i <= n; i++)
        visited[i] = false;

    int cnt = 0;

    for (int i = n - 1; i >= 0; i--)
    {
        int node = order[i];
        if (!visited[node])
        {
            cnt++;
            dfs(node, cnt); //using revadj here.
            // cout << endl;
        }
    }

    if(cnt == 1){
        cout << "YES" << endl;
        return;
    }

    int u = -1, v = -1;
    for (int i = 1; i <= n; i++)
    {
        if (leaders[i] == 1)
            u = i;
        else if (leaders[i] == 2)
            v = i;
    }
    cout << "NO\n";
    cout << v << " " << u << "\n";
}

signed main()
{

    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    int _t = 1;

    // cin >> _t;

    while (_t--)
    {
        solve();
    }

    return 0;
}

/*
scc1 --> scc2 --> scc3
so we cannot ever go from an element with leader i to an element with leader (i-1).
*/