时间复杂度:O(nm)

void SPFA()
{
    queue<int>q;
    memset(d,0x3f,sizeof d);
    memset(v,0,sizeof v);
    d[1] = 0;
    v[1] = 1;
    q.push(1);               
    while(q.size())
    {
        int x = q.front();
        q.pop();
        v[x] = 0;
        for(int i = head[x]; i ; i = Next[i])
        {
            int y = ver[i];
            int z = edge[i];
            if(d[y] > d[x] + z)
            {
                d[y] = d[x] + z;
                if(!v[y])q.push(y),v[y] = 1;
            }
        }
    }
}