SPFA算法之二

//当题目中并没有明确的唯一源点，就需要添加超级源点,它到各顶点都可达，
//因为如果图是非连通的，那么有些顶点将一直得不到检查，
//解决方法就是在初始队列时，把所有的顶点都压入队列里


#include <iostream>        //poj 2983 差分约束系统，使用并查集
#include <queue>
using namespace std;

const int MAX = 500000;
const int INF = 1000000000;
const int N = 2000;

struct Node
{
    int v;
    int cost;
    int next;
};

Node node[MAX];
int cnt[N];
int adj[N];
bool in_q[N];
int d[N];
int size;
int n, m;

void add_edge(int u, int v, int cost)
{
    node[size].v = v;
    node[size].cost = cost;
    node[size].next = adj[u];
    adj[u] = size++;
}

bool spfa()
{
    queue<int> Q;

    memset(cnt, 0, sizeof(cnt));
    memset(in_q, false, sizeof(in_q));
    for (int i = 0; i <= n; i++)
        d[i] = INF;
    d[0] = 0;
    int u,v,w;
    in_q[0] = true;


    for(int i=1;i<=n;++i)
    Q.push(i);


    while (!Q.empty())
    {
        u = Q.front();
        Q.pop();
        in_q[u] = false;

        for (int i = adj[u]; i != -1; i = node[i].next)
        {
            v = node[i].v;
            w = node[i].cost;

            if (d[v] < d[u]+w)
            {
                d[v] = d[u] + w;
                if (!in_q[v])
                {
                    in_q[v] = true;
                    Q.push(v);

                    if (++cnt[v] >= n) return false;
                }
            }
        }
    }
    return true;
}
int main()
{
    char c;
    int a, b ,w;

    while (scanf("%d%d", &n, &m) != EOF)
    {
        size = 0;
        for (int i = 0; i <= n; i++)
            adj[i] = -1;

        for (int i = 0; i < m; i++)
        {
            getchar();
            scanf("%c", &c);
            if (c == 'P')
            {
                scanf("%d%d%d", &a, &b, &w);
                add_edge(b, a, w);
                add_edge(a, b, -w);
            }
            else if (c == 'V')
            {
                scanf("%d%d", &a, &b);
                add_edge(b, a, 1);
            }
        }                
    if (spfa())
        printf("Reliable\n");
    else
        printf("Unreliable\n");
    }
    return 0;
}