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Discussion of Problem 1930. Ivan's Car

I will just post the soution because i want to ruin it for you ;)
Posted by Ishan Pandey 18 Mar 2020 10:00
#include<bits/stdc++.h>
using namespace std;
#define endl "\n"
#define int long long
#define mod 1000000007

#define maxn 10010
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define inf 100000000000

vector<pair<int, int>> adj[maxn];
int n, m;

// struct node //for edges
// {
//     int pos, dir, dist;

//     node(){};
//     node(int _pos, int _dir)
//     {
//         pos = _pos;
//         dir = _dir;
//     }
// };

int go(int s, int e)
{

    int dist[maxn][2];
    for(int i=0; i<maxn; i++)
    {
        for(int j=0; j<=1; j++)
        {
            dist[i][j] = inf;
        }
    }

    deque<pair<int, int>> q; //pair->{node, edgedir}

    //making direction list here,
    q.push_back({s, 0}); //0->outgoing to node s
    dist[s][0] = 0;
    q.push_back({s, 1}); //1->incoming from node s
    dist[s][1] = 0;

    while(!q.empty())
    {
        pair<int, int> cur = q.front();
        //node
        int v = cur.first;
        int dir = cur.second;
        //dist
        int dv = dist[v][dir];

        //guarenteed that shortest dist till cur node has been calc already

        q.pop_front();
        //if(dv == inf) break;

        if(cur.first == e) return dv; //gauarenteed via djikstra that this willbe minimest

        //relax all the neighbours, and as you relax one , push into minheap the nodes
        for(auto it: adj[v])
        {
            int to = it.fi;
            int dir2 = it.se;
            //previous
            //note that for type edge dir, you have relax for all neighbouring nodes
            int dist2=0;
            if(dir2 == dir)
            {
                dist2 = dist[v][dir2];
                //dist[to][dir2] = min( dist[to][dir2], dist[v][dir2] );
            }
            else dist2 = dist[v][dir] + 1;
               // dist[to][dir2] = min( dist[to][dir2], dist[v][dir] + 1);


            //only relax and add to queue, if less than prev distance to this node
            if(dist2 < dist[to][dir2])
            {
                if(dist2 <= dv) //less than equal to cur_dist
                    q.push_front({to, dir2});
                else
                    q.push_back({to, dir2});

                dist[to][dir2] = dist2;

            }


        }
    }

    return -1;


}

void solve()
{
    cin>>n>>m;

    for(int i=1;i<=m;i++)
    {
        int x, y; cin>>x>>y;
        adj[x].pb({y, 0});
        adj[y].pb({x, 1});
    }
    int s, e; cin>>s>>e;
    //cout<<"hi";
    int ans = go(s, e);
    cout<<ans<<endl;


}

signed main()
{
    int t=1;
    //cin>>t;
    while(t--)
    {
        solve();
    }
    return 0;
}