1 #include<stdio.h>
  2 #include<string.h>
  3 #define MAXN 101
  4 struct edge{
  5     int v,e,l,time;
  6 };
  7 struct adjs{
  8     struct edge ed[MAXN];
  9     struct edge ed2[MAXN];
 10     int len;
 11     int count;
 12 };
 13 struct adjs adj[MAXN];
 14 int ve[MAXN],vl[MAXN],seq[MAXN],start,end;
 15 int m,n,indegree[MAXN];
 16 void read();
 17 int tops();
 18 int main(){
 19     int i,j,v1,v2,time,etime;
 20     read();
 21     etime = tops();
 22     if(etime!=-1){
 23         printf("%d\n",etime);
 24         for(i=1;i<=n;i++){
 25             v1 = i;
 26             for(j=adj[i].count-1;j>=0;j--){
 27                 v2 = adj[v1].ed[j].v;
 28                 time = adj[v1].ed[j].time;
 29                 adj[v1].ed[j].l = vl[v2]-time;
 30                 adj[v1].ed[j].e = ve[v1];
 31                 if(adj[v1].ed[j].l==adj[v1].ed[j].e){
 32                     printf("%d->%d\n",v1,v2);
 33                 }
 34             }
 35         }
 36         
 37     }
 38     else printf("0\n");
 39     return 0;
 40 }
 41 void read(){
 42     scanf("%d %d",&n,&m);
 43     int i,j,v1,v2,lastime;
 44     for(i=0;i<MAXN;i++) {
 45         adj[i].count=0;
 46         adj[i].len=0;
 47     }
 48     for(i=0;i<=n;i++) indegree[i]=0;
 49     adj[0].count = m;
 50     for(i=1;i<=m;i++){
 51         scanf("%d %d %d",&v1,&v2,&lastime);
 52         adj[v1].ed[adj[v1].count].v = v2;
 53         adj[v1].ed[adj[v1].count].time = lastime;
 54         adj[v1].ed[adj[v1].count].e=0;
 55         adj[v1].ed[adj[v1].count].l=0;
 56         adj[v1].count++;    
 57         indegree[v2]++;
 58         
 59         adj[v2].ed2[adj[v2].len].v=v1;
 60         adj[v2].ed2[adj[v2].len].time=lastime;
 61         adj[v2].len++;
 62     }
 63 }
 64 int tops(){
 65     int i,j,value,v1,v2,time,etime;
 66     for(i=1;i<=n;i++){
 67         ve[i] = 0;
 68     }
 69     start=0;
 70     end =0;
 71     for(i=1;i<=n;i++)
 72         if(indegree[i]==0) seq[end++]=i;
 73     
 74     //ve计算 
 75     while(start<end){
 76         v1 = seq[start];
 77         start++;
 78         for(i=0;i<adj[v1].count;i++){
 79             v2 = adj[v1].ed[i].v;
 80             time = adj[v1].ed[i].time;
 81             indegree[v2]--;
 82             if(indegree[v2]==0) seq[end++] = v2;
 83             if((ve[v1]+time)>ve[v2]){
 84                 ve[v2] = ve[v1]+time;
 85             }
 86         }
 87     }
 88     
 89     if(end!=n) return -1;
 90     etime = 0;
 91     for(i=1;i<=n;i++){
 92         if(ve[i]>etime) etime = ve[i];
 93     } 
 94     //vl 计算c
 95     for(i=1;i<=n;i++) vl[i] = etime;
 96     start = end-1;
 97     while(end>=0){
 98         v2 = seq[end--];
 99         for(i=0;i<adj[v2].len;i++){
100             v1 = adj[v2].ed2[i].v;
101             time = adj[v2].ed2[i].time;
102             if(vl[v1]>(vl[v2]-time)){
103                 vl[v1] = vl[v2]-time;
104             }
105         }
106     }
107     return etime;
108 }

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