Longest_Flight_Route.md

Algorithm: Data Structure

Problem: Longest Flight Route

#include <bits/stdc++.h>
using namespace std;
//#include<ext/pb_ds/assoc_container.hpp>
//#include<ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds; // find_by_order, order_of_key
#define ll long long
#define vi vector<int>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define vvi vector<vi>
#define vll vector<ll>
#define vvll vector<vector<ll>>
#define vpii vector<pair<int,int>>
#define vpll vector<pair<ll,ll>>
#define vb vector<bool>
#define vs vector<string>
///............x...........///
#define all(a) a.begin(), a.end()
#define allr(a) a.rbegin(), a.rend()
#define mp(a, b) make_pair(a, b)
#define pb push_back
#define ff first
#define ss second
#define bg begin()
#define UNIQUE(X) (X).erase(unique(all(X)), (X).end())
#define ft cout << "for test"<<endl;
#define read(v, a, n) for (int i = a; i<n; i++)cin>>v[i];
#define print(v) for (auto it : v)cout << it<<" ";cout << endl;
#define PI acos(-1.0)
#define yes cout <<"Yes"<< endl
#define no cout<<"No"<<endl
#define FIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define t_c int test, cs = 1;cin>>test;while (test--)
#define casep cout<<"Case "<< cs++<<": ";
///................function.....................///
 
#define D(a) (double)(a)
#define siz(s) (int)(s.size())
#define max3(a, b, c) max(a, max(b, c))
#define min3(a, b, c) min(a, min(b, c))
#define Min(a) *min_element(all(a))
///#define mod 1000000007
///.........Graph.........///
int X[] = {1, -1, 0, 0};
int Y[] = {0, 0, 1, -1};
///........constant........///
const ll N = 1e6+5;
void file(){
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
}
#define inf (1ll<<62)
vvi adj;
vi parent,dis,in;
int n;
void bfs(queue<int>& q, bool type=0){
    while(!q.empty()){
        int u = q.front(); q.pop();
        if(!type and u==1)continue;
        // cout<<"u "<<u<<endl;
        for(auto x:adj[u]){
            in[x]--;
            if(in[x]==0){
                parent[x]=u;
                q.push(x);
            }
        }
    }
}
int main(){
    FIO;
    // file();
    int m,i,j;
    cin>>n>>m;
    adj.resize(n+1);
    dis.resize(n+1);
    in.resize(n+1);
    parent.resize(n+1);
    for(i=0; i<m; i++){
        int a,b;
        cin>>a>>b;
        adj[a].pb(b);
        in[b]++;
    }
    queue<int>q;
    for(i=2; i<=n; i++){
        if(in[i]==0)q.push(i);
    }
    bfs(q);
    // cout<<endl<<"done"<<endl;
    q.push(1);
    bfs(q,1);
    vi ans;
    int p = n;
    while(true){
        ans.pb(p);
        if(p<=1)break;
        p = parent[p];
    }
    reverse(all(ans));
    if(ans.front()==1 and ans.back()==n){
        cout<<ans.size()<<endl;
        print(ans);
    }
    else cout<<"IMPOSSIBLE"<<endl;
}