POJ 3255 Roadblocks

求次短路

 

#include <iostream>

#include <cstdlib>

#include <cstdio>

#include <algorithm>

#include <vector>

#include <queue>

#include <cmath>

#include <cstring>

using namespace std;

#define INF 0xfffffff

#define maxn 5060



struct Edge

{

    int e, w;

    Edge(int e=0,int w=0): e(e), w(w) {}

};



int n, m;

int dist[2][maxn];

int vis[maxn];



vector<Edge> G[maxn];



void  Spfa(int Star,int k)

{

    Edge P, Pn;

    queue <Edge> Q;

    dist[k][Star] = 0;

    Q.push( Edge(Star,0) );



    while( !Q.empty() )

    {

        P = Q.front();

        Q.pop();

        vis[P.e] = false;

        int len = G[P.e].size();



        for(int i=0; i<len; i++)

        {

            Pn = G[P.e][i];



            if(dist[k][Pn.e] > dist[k][P.e] + Pn.w )

            {

                dist[k][Pn.e] = dist[k][P.e] + Pn.w;



                if(!vis[Pn.e] )

                {

                    Q.push(Pn);

                    vis[Pn.e] = true;

                }

            }

        }

    }

}

int Slove()

{

    int ans = INF;

    int Min = dist[0][n];

    Edge P;

    for(int i=1; i<=n; i++)

    {

        int len = G[i].size();



        for(int j=0; j<len; j++)

        {

            P = G[i][j];

            int temp = dist[0][i] + dist[1][P.e] + P.w;

            if(temp > Min && temp < ans)

                ans = temp;

        }

    }

    return ans;

}

void Init()

{

    for(int i=1; i<=n; i++)

    {

        G[i].clear();

        vis[i] = false;

        dist[0][i] = dist[1][i] = INF;

    }

}

int main()

{

    while(cin >> n >> m)

    {

        int a, b, c;

        Init();

        for(int i=0; i<m; i++)

        {

            scanf("%d%d%d",&a,&b,&c);

            G[a].push_back( Edge(b,c) );

            G[b].push_back( Edge(a,c) );

        }



        Spfa(1,0);

        Spfa(n,1);



        int ans = Slove();



        cout << ans << endl;



    }

    return 0;

}