graph/Message Route at master · jt112/graph · GitHub

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//g++  5.4.0

#include<bits/stdc++.h>
using namespace std;

#define ff first
#define ss second
#define int long long
#define pb push_back
#define pii pair< int,int >
#define fast ios::sync_with_stdio(0) , cin.tie(0) , cout.tie(0) ;

const int nax = 1e5+5;
vector<int > g[nax] , par(nax,0);

signed main()
{
    fast ;
    int n , m ;
    cin >> n >> m ;

    for(int i=0 ; i<m ; i++ )
    {
        int u , v;
        cin >> u >> v ;
        g[u].pb(v);
        g[v].pb(u);
    }
    queue < int > q;
    q.push( 1 );
    par[1] = -1;
    while( !q.empty() )
    {
        int p = q.front() ;
        q.pop();
        if( p == n ) break;
        for( auto &i : g[p] )
            if( !par[i] ) par[i] = p , q.push( i );
    }
    if( par[n] )
    {
        stack<int> s;
        s.push( n );
        int p = par[n] ;
        while( p != -1 )
        {
            s.push( p );
            p = par[p];
        }
        cout << (int)(s.size() ) << "\n";
        while( !s.empty() )
        {
            cout << s.top() << " " ;
            s.pop();
        }
    }
    else
        cout << "IMPOSSIBLE";

}