洛谷 P1807 最长路_NOI导刊2010提高（07）

最长路

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <vector>
using namespace std;
//Mystery_Sky
//
#define maxn 1000010
#define maxm 5000050
#define INF 0x3f3f3f3f
queue <int> q;
int ind[maxn], dis[maxn];
struct Edge{
	int to, next, w;
}edge[maxn];
int head[maxn], cnt, ans;
int n, m;
inline void add_edge(int u, int v, int w)
{
	edge[++cnt].to = v;
	edge[cnt].w = w;
	edge[cnt].next = head[u];
	head[u] = cnt;
}
inline void topo()
{
	for(int i = 1; i <= n; i++) {
		if(!ind[i]) {
			q.push(i);
		}
	}
	dis[1] = INF;
	while(!q.empty()) {
		int u = q.front();
		q.pop();
		for(int i = head[u]; i ; i = edge[i].next) {
			int v = edge[i].to;
//			printf("%d %d %d\n", u, v, edge[i].w);
			ind[v]--;
			dis[v] = max(dis[v], dis[u] + edge[i].w);
			if(!ind[v]) q.push(v);
		}
	}
}
int u, v, w;
int main() {
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= m; i++) {
		scanf("%d%d%d", &u, &v, &w);
		add_edge(u, v, w);
		ind[v]++;
	}
	topo();
	if(dis[n]) printf("%d\n", dis[n]-INF);
	else printf("-1\n");
	return 0;
}