【PTA】关键活动 (30 分)

#include <iostream>
#include <vector>
#include <stack>
#include <queue>
#define MAXN 110
using namespace std;

struct Node
{
    int v;
    int weight;
    Node(int _v,int _weight):v(_v),weight(_weight){}
};

int N,M;
vector<Node> Adj[MAXN];

stack<int> TopOrder;
int vl[MAXN],ve[MAXN];
int Indegree[MAXN];
int TopSort(){
    int LeastTime=0;
    queue<int> q;
    fill(ve,ve+N,0);
    for(int i=0;i<N;i++){
        if(Indegree[i]==0){
            q.push(i);
        }
    }
    while(!q.empty()){
        int u=q.front();
        q.pop();
        TopOrder.push(u);//保存了序列,就不需要再用cnt了

        for(int i=0;i<Adj[u].size();i++){
            int v=Adj[u][i].v;
            if(--Indegree[v]==0){
                q.push(v);
            }
            if(ve[u]+Adj[u][i].weight>ve[v]){
                ve[v]=ve[u]+Adj[u][i].weight;
            }
            if(ve[v]>LeastTime) LeastTime=ve[v];
        }

    }
    if(TopOrder.size()!=N) return 0;
    else       return LeastTime;
}

void CriticalPath(){
    int LeastTime=TopSort();
    printf("%d\n",LeastTime);
    if(LeastTime==0) return;
    fill(vl,vl+N,LeastTime);

    while(!TopOrder.empty()){
        int u=TopOrder.top();
        TopOrder.pop();

        for(int i=0;i<Adj[u].size();i++){
            int v=Adj[u][i].v;

            if(vl[v]-Adj[u][i].weight<vl[u]){
                vl[u]=vl[v]-Adj[u][i].weight;
            }
        }
    }

    for(int u=0;u<N;u++){
        for(int i=Adj[u].size()-1;i>=0;i--){//边起点升序,边终点按输入逆序
            int v=Adj[u][i].v;
            int w=Adj[u][i].weight;

            int e=ve[u];
            int l=vl[v]-w;
            if(e==l){
                printf("%d->%d\n",u+1,v+1);
            }
        }
    }

}



int main(){
    scanf("%d %d",&N,&M);
    
    int tmp1,tmp2,tmpl;
    for(int i=0;i<M;i++){
        scanf("%d %d %d",&tmp1,&tmp2,&tmpl);
        Adj[tmp1-1].push_back(Node(tmp2-1,tmpl));
        Indegree[tmp2-1]++;//入度在输入时初始化
    }

    CriticalPath();
   
    return 0;
}