RT,$5,6$T了怀疑写假了=.=

#include<bits/stdc++.h>
using namespace std;
const int maxn=1505;
const int inf=99999999;
inline int read()
{
    int x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}
    return x*f;
}
int n,m,start;
queue<int>Q;
int dis[maxn];
bool vis[maxn];
int head[maxn];
int tot=0;
struct node
{
    int to,nxt,c;
}edge[maxn];
inline void add(int u,int v,int c)
{
    edge[++tot].to=v;
    edge[tot].c=c;
    edge[tot].nxt=head[u];
    head[u]=tot;
}
inline void SPFA()
{
    for(int i=1;i<=n;i++)dis[i]=inf,vis[i]=0;
    dis[1]=0,vis[1]=1,Q.push(1);
    while(!Q.empty())
    {
        int u=Q.front();Q.pop();vis[u]=0;
        for(int i=head[u];i;i=edge[i].nxt)
        {
            int v=edge[i].to;
            if(dis[v]>dis[u]+edge[i].c)
            {
                dis[v]=dis[u]+edge[i].c;
                if(!vis[v])vis[v]=1,Q.push(v);
            }
        }
    }
}
int main()
{
    n=read();m=read();
    for(int i=1;i<=m;i++)
    {
        int u,v,c;
        u=read();v=read();c=read();
        add(u,v,-c);
    }
    SPFA();
    if(dis[n]==inf)return puts("-1")&0;
    printf("%d\n",-dis[n]);
}

方法就是边取反然后跑最短路，最后反过来输出就好了。