UVa 10986 Sending email

SPFA，2RE，无向图的边数为题目描述的两倍。

# include <cstdio>
# include <cstring>
# include <queue>

using namespace std;

# define N 20000 + 5
# define M (50000 + 5) * 2
# define INF 550000000

int n, m, src, des;

int first[N], d[N];
bool inq[N];
int u[M], v[M], w[M], next[M];

void read_graph(void)
{
    scanf("%d%d%d%d", &n, &m, &src, &des);
    memset(first, -1, sizeof(first[0])*n);
    for (int e = 0; e < 2*m; e += 2)
    {
        scanf("%d%d%d", &u[e], &v[e], &w[e]);
        next[e] = first[u[e]], first[u[e]] = e;
        u[e+1] = v[e], v[e+1] = u[e], w[e+1] = w[e];
        next[e+1] = first[u[e+1]], first[u[e+1]] = e+1;
    }
}

void spfa(int src, int n)
{
    queue <int> Q;
    
    memset(inq, false, sizeof(inq[0])*n);
    for (int i = 0; i < n; ++i) d[i] = (i==src ? 0:INF);
    Q.push(src);
    while (!Q.empty())
    {
        int x = Q.front(); Q.pop(); inq[x] = false;
        for (int e = first[x]; e != -1; e = next[e]) if (d[x] < INF && d[x]+w[e] < d[v[e]])
        {
            d[v[e]] = d[x] + w[e];
            if (inq[v[e]] == false)
            {
                inq[v[e]] = true;
                Q.push(v[e]);
            }
        }
    }
}

int main()
{
    int T;
        
    scanf("%d", &T);
    for (int i = 1; i <= T; ++i)
    {
        printf("Case #%d:", i);
        read_graph();
        spfa(src, n);
        if (d[des] >= INF) puts(" unreachable");
        else printf(" %d\n", d[des]);
    }
    
    return 0;
}