最长路SPFA写法

#include <bits/stdc++.h>
#define int long long//开long long 
using namespace std;
const int N=5e4+5;
const int inf=2147483647;
int n,m;
struct edge{
	int to;
	int w;
};
vector<edge>G[N];
bool vis[N];
int dis[N];
void spfa(int s){//有正边权用SPFA 
	for(int i=1;i<=n;i++)dis[i]=-inf;//注意:最长路要初始化为无穷小 
	queue<int>q;
	vis[s]=1;
	dis[s]=0;
	q.push(s);//初始化 
	while(!q.empty()){
		int now=q.front();
		q.pop();
		vis[now]=0;//now不在队列中 
		for(auto x:G[now]){
			int nex=x.to;
			int val=x.w;
			if(dis[now]+val>dis[nex]){//注意:最长路用">" 
				dis[nex]=dis[now]+val;
				if(vis[nex])continue;
				vis[nex]=1;
				q.push(nex);
			}
		}
	}
}	
signed main(){
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int u,v,w;
		cin>>u>>v>>w;
		G[u].push_back({v,w});
	}
	spfa(1);
	if(dis[n]==-inf)cout<<-1;//如果为负无穷,输出-1 
	else cout<<dis[n];
	return 0;
}