#include<iostream>
#include<fstream>

using namespace std;

struct Edge
{
	int s;
	int e;
	int t;
};

static const int MAX = 30000;
static const int MAXEDGES = 6000;  /* caution */
static const int MAXNODES = 501;
static int n, m, w;
struct Edge edges[MAXEDGES];
static int dist[MAXNODES];

//#define DEBUG

int bellman_ford(void)
{
	int edgenum = m + m + w;  /* edges */
	int i, j;
	for (i = 1; i <= n; i++)
	{
		dist[i] = MAX;
	}
	dist[1] = 0;
	for (i = 1; i < n; i++) /* Relax all edges |v|-1 times */
	{
		int ok = 1;   /* reduce about 80ms */
		for (j = 0; j < edgenum; j++)
		{
			int s, e, t;
			s = edges[j].s;	e = edges[j].e;	t = edges[j].t;
			if (dist[s] + t < dist[e])
			{	dist[e] = dist[s] + t;	ok = 0; }
		}
		if (ok)
			break;
	}
	for (j = 0; j < edgenum; j++) /* check minus loop */
	{
		int s, e, t;
		s = edges[j].s;	e = edges[j].e;	t = edges[j].t;
		if (dist[s] + t < dist[e])
				return 0;
	}
	return 1;
}

int main()
{
#ifdef DEBUG
	fstream cin("G:\\book\\algorithms\\acm\\Debug\\dat.txt");
#endif

	int fields;
	cin >> fields;
	while (fields-- >0)
	{
		cin >> n >> m >> w;
		int i, k = 0;
		int s, e, t;
		for (i = 0; i < m; i++)
		{
			cin >> s >> e >>t;
			edges[k].s = s;
			edges[k].e = e;
			edges[k++].t = t;

			edges[k].s = e;
			edges[k].e = s;
			edges[k++].t = t;
		}
		for (i = 0; i < w; i++)
		{
			cin >> s >> e >>t;
			edges[k].s = s;
			edges[k].e = e;
			edges[k++].t = -t;  /* minus weigh edge */
		}
		if (!bellman_ford())
			cout << "YES" << endl;
		else
			cout << "NO" << endl; 
	}

	return 0;
}
 

演算法的正確性見 演算法導論 24.1節相關內容。

存在負權迴路，負權迴路上的頂點可以進行無限制的鬆弛。

存在負權路徑無法進行最短路徑的計算見 演算法導論 圖24 -1即可明白。

程式碼中加註釋的地方，都曾經出現過錯誤，注意。