64分求助

#include<iostream>
#include<cstring>
using namespace std;
const int N=1510,M=50010;
int f[N],fir[N],nex[N],en[M],len[M],d[100*M],r;
bool b[N];
void rd(int v,int dis){
	f[v]=dis;
	if(!b[v]){
		d[++r]=v;
		b[v]=true;
	}
}
int main(){
	int n,m;
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int u,v,w;
		scanf("%d%d%d",&u,&v,&w);
		nex[i]=fir[u];
		fir[u]=i;
		en[i]=v;
		len[i]=w;
	}
	memset(f,-63,sizeof(f));
	int inf=f[0];
	int h=1;
	rd(1,0);
	while(h<=r){
		int u=d[h];
		int t=fir[u];
		while(t>0){
			int v=en[t];
			int L=f[u]+len[t];
			if(f[v]<L)rd(v,L);
			t=nex[t];
		}
		h++;
		b[u]=false;
	}
	if(f[n]==inf)cout<<-1<<endl;
	else cout<<f[n]<<endl;
}