Sunday 18 June 2017

Almost Shortest Path (UVa 12144, UVaLive 4210, Regionals 2008 >> Latin America - South America) 


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///     Raihan Ruhin
///     CSE, Jahangirnagar University.
///     Dhaka-Bangladesh.
///     id: raihanruhin (topcoder / codeforces / codechef / uva ), 3235 (lightoj)
///     mail: raihanruhin@ (yahoo / gmail / facebook)
///     blog: ruhinraihan.blogspot.com

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

#define SET(a) memset(a,-1,sizeof(a))
#define CLR(a) memset(a,0,sizeof(a))
#define PI acos(-1.0)

#define MOD 1000000007
#define MX 502

struct Z
{
    int v, cost;
    bool operator < (const Z &p) const
    {
        return p.cost<cost;
    }
};

Z z, z2;

int shortest[MX], forbidden[MX][MX];
vector<Z> adj[MX];
vector<int> parent[MX];
priority_queue<Z>pq;

int dijkstra(int src, int dst, int node)
{
    for(int i=0; i<node; i++)
        shortest[i]=1e9;

    z.v = src;
    z.cost = 0;
    pq.push(z);
    shortest[src]=0;

    while(!pq.empty())
    {
        z = pq.top();
        pq.pop();

        int l = adj[z.v].size();

        for(int i=0; i<l; i++)
        {
            z2 = adj[z.v][i];
            if(forbidden[z.v][z2.v]==1) continue;
            if(shortest[z2.v]>shortest[z.v]+z2.cost)
            {
                parent[z2.v].clear();
                parent[z2.v].push_back(z.v);
                z2.cost = shortest[z2.v]= shortest[z.v]+z2.cost;
                pq.push(z2);
            }
            else if(shortest[z2.v]==shortest[z.v]+z2.cost)
                parent[z2.v].push_back(z.v);

        }
    }
    return shortest[dst];
}

void removeEdge(int src, int dst)
{
    if(dst==src) return;
    for(int i=0;i<parent[dst].size();i++)
    {
        forbidden[ parent[dst][i] ][dst] = 1;
        removeEdge(src, parent[dst][i]);
    }
return;
}

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int tc,kk=1, node, edge, u, v, cost, src, dst;

    while(cin>>node>>edge && node)
    {
        cin>>src>>dst;


        for(int i=0; i<edge; i++)
        {
            cin>>u>>v>>cost;

            z.v = v;
            z.cost = cost;
            adj[u].push_back(z);
        }

        int tmp = 1e9;
        int mn = dijkstra(src, dst, node); //initial

        if(mn<1e9)
        {
            removeEdge(src, dst); //remove edges of shortest
            tmp = dijkstra(src, dst, node); //second time
        }
        if(tmp==1e9) tmp = -1;
        cout<<tmp<<endl;


        CLR(forbidden);
        for(int i=0;i<node;i++)
        {
            adj[i].clear();
            parent[i].clear();
        }
    }
    return 0;
}
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