UVA 558 Wormholes

SPFA判环。

CODE：

 

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

#define MAXN 1100
#define INF 0X3F3F3F3F

struct Edge
{
    int u, v, w;
    int next;
}edge[MAXN*4];

int first[MAXN], d[MAXN];
int n, m, cnt;

void init()
{
    cnt = 0;
    memset(first, -1, sizeof(first));
}

int spfa(int src)
{
    queue<int> q;
    bool inq[MAXN] = {0};
    int count[MAXN] = {0};
    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] = 0;
        for(int e = first[x]; e != -1; e = edge[e].next)
        {
            int v = edge[e].v, w = edge[e].w;
            if(d[v] > d[x] + w)
            {
                d[v] = d[x] + w;
                if(!inq[v])
                {
                    inq[v] = 1;
                    if(++count[v] > n) return 0; //判环 
                    q.push(v);
                }
            }
        }
    }
    return 1;
}

void read_graph(int u, int v, int w)
{
    edge[cnt].v = v, edge[cnt].w = w;
    edge[cnt].next = first[u], first[u] = cnt++;
}

int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        init();
        scanf("%d%d", &n, &m);
        while(m--)
        {
            int u, v, w;
            scanf("%d%d%d", &u, &v, &w);
            read_graph(u, v, w);
        }
        if(!spfa(0))
        {
            printf("possible\n");
        }
        else
        {
            printf("not possible\n");
        }
    }
}