POJ 3268 Silver Cow Party

题意:给定顶点个数，边的个数，目的地，求顶点到边的往返最大距离

解题思路：目的地到顶点的是标准的Dijkstra，顶点到目的地的只需要把矩阵转置一下就OKay了

#include <iostream>
#include <cstring>
#include <algorithm>
#define MAXVERTEXNUM 1005
#define INF 1000000
using namespace std;

int Nv, Ne, Des;
int MGraph[MAXVERTEXNUM][MAXVERTEXNUM];
int dist1[MAXVERTEXNUM], dist2[MAXVERTEXNUM];

void Dijkstra()
{
    int collected[MAXVERTEXNUM];
    memset(collected, 0, sizeof(collected));
    collected[Des] = 1;
    dist1[Des] = 0;

    for (int i = 1; i <= Nv; ++i)
    {
        if (MGraph[Des][i] != INF)
            dist1[i] = MGraph[Des][i];
        else
            dist1[i] = INF;
    }

    for (int i = 2; i <= Nv; ++i)
    {
        int minDis = INF, num;
        for (int j = 1; j <= Nv; ++j)
            if (!collected[j] && minDis > dist1[j])
            {
                minDis = dist1[j];
                num = j;
            }

        collected[num] = 1;
        dist1[num] = minDis;

        for (int j = 1; j <= Nv; ++j)
            if (!collected[j] && dist1[j] > dist1[num] + MGraph[num][j])
                dist1[j] = dist1[num] + MGraph[num][j];
    }
}

void T()
{
    for (int i = 1; i <= Nv; ++i)
        for (int j = 1; j <= i; ++j)
            swap(MGraph[i][j], MGraph[j][i]);
}

int main()
{
    ios::sync_with_stdio(false);

    cin >> Nv >> Ne >> Des;

    for (int i = 1; i <= Nv; ++i)
        for (int j = 1; j <= Nv; ++j)
            if (i != j)
                MGraph[i][j] = INF;
            else
                MGraph[i][j] = 0;

    for (int i = 1; i <= Ne; ++i)
    {
        int V1, V2, Edge;
        cin >> V1 >> V2 >> Edge;
        MGraph[V1][V2] = Edge;
    }

    Dijkstra();
    for (int i = 1; i <= Nv; ++i)
        dist2[i] = dist1[i];
    T();
    Dijkstra();

    int maxDis = 0;
    for (int i = 1; i <= Nv; ++i)
        if (i != Des)
        {
            int temp = 0;
            temp += dist1[i];
            temp += dist2[i];
            if (temp > maxDis)
                maxDis = temp;
        }

    cout << maxDis << endl;

    return 0;
}