08-图8 How Long Does It Take （25 分

Given the relations of all the activities of a project, you are supposed to find the earliest completion time of the project.

Input Specification:

Each input file contains one test case. Each case starts with a line containing two positive integers N (≤), the number of activity check points (hence it is assumed that the check points are numbered from 0 to N−1), and M, the number of activities. Then M lines follow, each gives the description of an activity. For the i-th activity, three non-negative numbers are given: S[i], E[i], and L[i], where S[i] is the index of the starting check point, E[i] of the ending check point, and L[i]the lasting time of the activity. The numbers in a line are separated by a space.

Output Specification:

For each test case, if the scheduling is possible, print in a line its earliest completion time; or simply output "Impossible".

Sample Input 1:

9 12
0 1 6
0 2 4
0 3 5
1 4 1
2 4 1
3 5 2
5 4 0
4 6 9
4 7 7
5 7 4
6 8 2
7 8 4

Sample Output 1:

18

Sample Input 2:

4 5
0 1 1
0 2 2
2 1 3
1 3 4
3 2 5

Sample Output 2:

Impossible

#include<cstdio>
#include<queue>
using namespace std;
const int maxn = ;
int map[maxn][maxn],d[maxn],indegree[maxn];
int main(){
    int n,m;
    scanf("%d%d",&n,&m);
    for(int i = ; i < n; i++){
        d[i] = -;
        indegree[i] = ;
        for(int j = ; j < n; j++){
            map[i][j] = map[j][i] = -;
        }
    }
    int u,v,w;
    for(int i = ; i < m; i++){
        scanf("%d%d%d",&u,&v,&w);
        map[u][v] = w;
        indegree[v]++;
    }
    queue<int> q;
    for(int i = ; i < n; i++){
        if(!indegree[i]){
            q.push(i);
            d[i] = ;
        }
    }
    int cur;
    while(!q.empty()){
        cur = q.front();
        q.pop();
        for(int i = ; i < n; i++){
            if(map[cur][i] != -){
                indegree[i]--;
                if(d[cur] + map[cur][i] > d[i]){
                    d[i] = d[cur] + map[cur][i];
                }
                if(!indegree[i]){
                    q.push(i);
                }
            }
        }
    }
    int maxCost = -;
    int i;
    for(i =  ; i < n; i++){
        if(indegree[i]) break;
        if(d[i] > maxCost) maxCost = d[i];
    }
    if(i == n) printf("%d",maxCost);
    else printf("Impossible");
    return ;
}