7-12 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 (≤100), 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

源码：

#include <stdio.h>
#include <stdlib.h>


#define INFINITY 99999999
#define MaxVertexNum 110 /* maximum number of vertices */
typedef int Vertex;      /* vertices are numbered from 0 to MaxVertexNum-1 */
typedef int WeightType;

typedef struct GNode *PtrToGNode;
struct GNode{
    int Nv;
    int Ne;
    WeightType G[MaxVertexNum][MaxVertexNum];
};
typedef PtrToGNode MGraph;
int pen[MaxVertexNum]={0};
int result[MaxVertexNum]={0};
int stack[MaxVertexNum];
int top=-1;
int least[MaxVertexNum]={0};

MGraph ReadG(int N,int M)
{
    MGraph G=(MGraph)malloc(sizeof(struct GNode));
    G->Nv=N;
    G->Ne=M;
    int i,j;
    for(i=0;i<G->Nv;i++)
    {
        for(j=0;j<G->Nv;j++)
        {
            G->G[i][j]=INFINITY;
            if(i==j)
            {
                G->G[i][j]=0;
            }
        }
    }
    for(i=1;i<=G->Ne;i++)
    {
        int a,b,x,y;
        scanf("%d%d%d",&a,&b,&x);
        G->G[a][b]=x;
        pen[b]++;
    }

    for(i=0;i<G->Nv;i++)
    {
        if(pen[i]==0)stack[++top]=i;
    }
    return G;
}
int topo(MGraph G)
{
    int m=0;
    while(top!=-1)
    {
        int t=stack[top--];
        m++;
        for(int i=0;i<G->Nv;i++)
        {
            if(G->G[t][i]!=INFINITY&&t!=i)
            {
                pen[i]--;
                if(result[t]+G->G[t][i]>result[i])result[i]=result[t]+G->G[t][i];
                if(pen[i]==0)stack[++top]=i;
            }
        }
    }
    if(m<G->Nv)return 0;
    else {
        int max=0;
        for(int i=0;i<G->Nv;i++)
        {
            if(result[i]>=max)max=result[i];
        }

        return max;
    }
}
int main()
{
    int N,M;
    scanf("%d%d",&N,&M);
    MGraph G=ReadG(N,M);
    int f=topo(G);

    if(f)
        printf("%d\n",f);
    else
        printf("Impossible\n");

    return 0;
}