PTA 关键活动 （30 分）

1.求最短时间和所有的关键活动，注意多起点多终点：https://blog.csdn.net/qq_40772692/article/details/84190698

2.代码：

#include <iostream>
#include<bits/stdc++.h>
using namespace std;
#define INF 0x3f3f3f3f
const int maxn = 100 + 7;
struct Edge{
    int from,to,next,time,id;
}edge[maxn*1000],edge2[maxn*1000];
int n,m,tot,tot2,head[maxn],in[maxn],ve[maxn],out[maxn],head2[maxn],vl[maxn],Max;
void addEdge(int a,int b,int c,int d){
    edge[tot].id = d;edge[tot].from = a;edge[tot].to = b;edge[tot].time = c;edge[tot].next = head[a],head[a] = tot++;
}
void addEdge2(int a,int b,int c){
    edge2[tot2].from = a;edge2[tot2].to = b;edge2[tot2].time = c;edge2[tot2].next = head2[a],head2[a] = tot2++;
}
struct Node{
   int s,e,id;
   bool operator <(const Node &another)const{
       if(s==another.s)return id < another.id;
       return s > another.s;
   }
   Node(int a,int b,int c):s(a),e(b),id(c) {}
};
bool TupoSort(){
   queue<int> que;
   memset(ve,0,sizeof(ve));
   for(int i = 1;i<=n;i++){
      if(in[i]==0)que.push(i);
      //cout<<ve[i]<<" "<<in[i]<<endl;
   }
   int cnt = 0;
   while(!que.empty()){
      cnt++;
      int p = que.front();
      que.pop();
      for(int i = head[p];~i;i = edge[i].next){
          in[edge[i].to]--;
          if(ve[edge[i].to] < ve[p] + edge[i].time){
              ve[edge[i].to] = ve[p] + edge[i].time;
          }
          if(!in[edge[i].to])que.push(edge[i].to);
      }
   }
   if(cnt!=n)return false;
   return true;
}
void ReTupoSort(){
   memset(vl,INF,sizeof(vl));
   queue<int> que;
   for(int i = 1;i<=n;i++){
      if(out[i]==0){
        que.push(i);
        vl[i] = Max;
      }
   }
   while(!que.empty()){
      int p = que.front();
      que.pop();
      for(int i = head2[p];~i;i = edge2[i].next){
         out[edge2[i].to]--;
         if(vl[edge2[i].to] > vl[p] - edge2[i].time){
            vl[edge2[i].to] = vl[p] - edge2[i].time;
         }
         if(!out[edge2[i].to])que.push(edge2[i].to);
      }
   }
}
int main()
{
    tot = tot2 = 0;
    memset(head,-1,sizeof(head));
    memset(head2,-1,sizeof(head2));
    memset(in,0,sizeof(in));
    memset(out,0,sizeof(out));
    scanf("%d%d",&n,&m);
    for(int i = 0;i<m;i++){
        int a,b,v;
        scanf("%d%d%d",&a,&b,&v);
        addEdge(a,b,v,i);
        addEdge2(b,a,v);
        in[b]++;
        out[a]++;
    }
    if(!TupoSort())printf("0\n");
    else{
       Max = -1;
       for(int i = 1;i<=n;i++){
          if(out[i]==0)Max = max(Max,ve[i]);
       }
       printf("%d\n",Max);
       ReTupoSort();
       priority_queue<Node> que;
       for(int i = 1;i<=n;i++){
          if(ve[i]!=vl[i])continue;
          for(int j = head[i];~j;j = edge[j].next){
             if(ve[i]==vl[edge[j].to] - edge[j].time){
                que.push(Node(i,edge[j].to,edge[j].id));
             }
          }
       }
       while(!que.empty()){
           Node node = que.top();
           que.pop();
           printf("%d->%d\n",node.s,node.e);
       }
    }
}