這個題目，有一個未說明的條件：如果c為負數，則將c修改為0。

#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <vector>
#include <deque>
#include <limits>
using namespace std;

namespace dts
{
    typedef long long ll;
    typedef unsigned long long ull;
    
    ull n,m;
    vector<ull> a,b,c,vis,cnt,dist;
    vector<vector<ull>> e,l;
    deque<ull> q;
    
    void bfs_resize(ull size)
    {
        vis.resize(size,0),cnt.resize(size,0);
        dist.resize(size,0);
        e.resize(size),l.resize(size);
        for (ull i=0;i<size;i++)
            e[i].clear(),l[i].clear();
        q.clear();
    }
    
    void add_edge(ull x,ull y,ull d)
    {
        e[x].push_back(y);
        l[x].push_back(d);
    }
    
    void bfs()
    {
        for (ull i=1;i<=n;i++)
            vis[i]=1,cnt[i]++,q.push_back(i);
        for (;!q.empty();vis[q.front()]=0,q.pop_front())
        {
            ull now=q.front();
            for (ull i=0;i<e[now].size();i++)
                if (dist[now]+l[now][i]>dist[e[now][i]])
                {
                    dist[e[now][i]]=dist[now]+l[now][i];
                    if (vis[e[now][i]]==0)
                    {
                        if (cnt[e[now][i]]>=n)
                            goto no_solution;
                        else
                        {
                            vis[e[now][i]]=1,cnt[e[now][i]]++;
                            q.push_back(e[now][i]);
                        }
                    }
                    else if (now==e[now][i])
                    {
                        no_solution:
                        printf("-1\n");
                        return;
                    }
                }
        }
        ull ans=0;
        for (ull i=1;i<=n;i++)
            ans+=dist[i];
        printf("%llu\n",ans);
    }
    
    void main()
    {
        while (~scanf("%llu%llu",&n,&m))
        {
            a.resize(m,0),b.resize(m,0),c.resize(m,0);
            for (ull i=0;i<m;i++)
            {
                char sa[1<<5],sb[1<<5],sc[1<<5];
                memset(sa,0,sizeof(sa)),memset(sb,0,sizeof(sb)),memset(sc,0,sizeof(sc));
                scanf("%s%s%s\n",sa,sb,sc);
                if (sc[0]=='-')
                    sscanf(sa,"%llu",&a[i]),sscanf(sb,"%llu",&b[i]),c[i]=0;
                else
                    sscanf(sa,"%llu",&a[i]),sscanf(sb,"%llu",&b[i]),sscanf(sc,"%llu",&c[i]);
            }
            bfs_resize(n+1);
            for (ull i=1;i<=n;i++)
                add_edge(0,i,0);
            for (ull i=0;i<m;i++)
                add_edge(a[i],b[i],c[i]);
            bfs();
        }
    }
}

int main()
{
    dts::main();
}