poj3268 Silver Cow Party

思路：

最短路。

实现：

#include <iostream>
#include <cstdio>
#include <vector>
#include <queue>
#include <algorithm>
#include <cstring>
using namespace std;

const int INF = 0x3f3f3f3f;
int d[1005], back[1005], times[1005];
bool in[1005];
struct edge
{
    int t, w;
    edge(int b, int c)
    {
        t = b; w = c;
    }
};
vector<edge> G[1005];
int n, m, x, a, b, l;

bool spfa(int s)
{
    for (int i = 1; i <= n; i++)
    {
        d[i] = INF;
        in[i] = false;
        times[i] = 0;
    }
    queue<int> q;
    q.push(s);
    in[s] = true;
    d[s] = 0;
    while (!q.empty())
    {
        int tmp = q.front();
        q.pop();
        in[tmp] = false;
        for (int i = 0; i < G[tmp].size(); i++)
        {
            if (d[tmp] + G[tmp][i].w < d[G[tmp][i].t])
            {
                d[G[tmp][i].t] = d[tmp] + G[tmp][i].w;
                times[G[tmp][i].t]++;
                if (times[G[tmp][i].t] == n)
                {
                    return false;
                }
                if (!in[G[tmp][i].t])
                {
                    in[G[tmp][i].t] = true;
                    q.push(G[tmp][i].t);
                }
            }
        }
    }
    return true;
}

int main()
{
    cin >> n >> m >> x;
    for (int i = 0; i < m; i++)
    {
        scanf("%d %d %d", &a, &b, &l);
        edge e(b, l);
        G[a].push_back(e);
    }
    int maxn = 0;
    spfa(x);
    memcpy(back, d, (n + 1) * sizeof(int));
    for (int i = 1; i <= n; i++)
    {
        spfa(i);
        maxn = max(maxn, d[x] + back[i]);
    }
    cout << maxn << endl;
    return 0;
}