这题#4、5、6运行错误，代码如下：

#include <cstdio>
#include <iostream>

using namespace std;

struct Edge {
	int to, next, dis;
};

int n, m, ans = 0;
int head[50001], way[100000][2];
Edge edge[1501];

void Bfs(int);
void AddEdge(int, int, int, int);

int main() {
	/*
	freopen("test.in", "r", stdin);
	freopen("test.out", "w", stdout);
	*/
	scanf("%d %d", &n, &m);
	for (int i=1; i<=m; i++) {
		int from, to, dis;
		scanf("%d %d %d", &from, &to, &dis);
		AddEdge(from, to, dis, i);
	}
	Bfs(1);
	if (ans == 0) {
		printf("-1\n");
	} else {
		printf("%d\n", ans);
	}
	/*
	fclose(stdin);
	fclose(stdout);
	*/
	return 0;
}

void Bfs(int start) {
	int left = 0, right = 1;
	way[1][0] = start;
	do {
		left++;
		for (int i=head[way[left][0]]; i>0; i=edge[i].next) {
			right++;
			way[right][0] = edge[i].to;
			way[right][1] = way[left][1]+edge[i].dis;
			if (edge[i].to == n) {
				ans = max(ans, way[right][1]);
			}
		}
	} while (left < right);
}

void AddEdge(int from, int to, int dis, int i) {
	edge[i].next = head[from];
	edge[i].to = to;
	edge[i].dis = dis;
	head[from] = i;
}

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