UVA 558 Wormholes

UVA_558

    这个题目实际上就是在让我们判断原图是否存在负圈，如果使用SPFA的话，当一个点的入队次数大于N时，就可以说明原图存在负圈了。

#include<stdio.h>
#include<string.h>
#define MAXD 1010
#define MAXM 2010
#define INF 1000000000
int d[MAXD], q[MAXD], inq[MAXD], inedq[MAXD];
int N, M, first[MAXD], e, next[MAXM], v[MAXM], w[MAXM];
void add(int a, int b, int c)
{
    v[e] = b;
    w[e] = c;
    next[e] = first[a];
    first[a] = e;
    e ++;
}
void init()
{
    int i, a, b, c;
    scanf("%d%d", &N, &M);
    memset(first, -1, sizeof(first));
    e = 0;
    for(i = 0; i < M; i ++)
    {
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
    }
}
int SPFA()
{
    int i, u, front, rear;
    front = rear = 0;
    for(i = 0; i < N; i ++)
        d[i] = INF;
    d[0] = 0;
    memset(inq, 0, sizeof(inq));
    memset(inedq, 0, sizeof(inedq));
    q[rear ++] = 0;
    while(front != rear)
    {
        u = q[front ++];
        if(front > N)
            front = 0;
        inq[u] = 0;
        for(i = first[u]; i != -1; i = next[i])
            if(d[u] + w[i] < d[v[i]])
            {
                d[v[i]] = d[u] + w[i];
                if(!inq[v[i]])
                {
                    q[rear ++] = v[i];
                    if(rear > N)
                        rear = 0;
                    inq[v[i]] = 1;
                    if(inedq[v[i]] ++ > N)
                        return 1;
                }
            }
    }
    return 0;
}
int main()
{
    int t;
    scanf("%d", &t);
    while(t --)
    {
        init();
        if(SPFA())
            printf("possible\n");
        else
            printf("not possible\n");
    }
    return 0;
}