ZOJ 1060 Sorting It All Out (topo-sort)

这就是人生中的第一道图论题？？？
   还是看别人题解做的。。。。。

Sorting It All Out

Time Limit : 2000/1000ms (Java/Other)   Memory Limit : 65536/32768K (Java/Other)

Total Submission(s) : 1   Accepted Submission(s) : 1

Problem Description

An ascending sorted sequence of distinct values is one in which some form of a less-than operator is used to order the elements from smallest to largest. For example, the sorted sequence A, B, C, D implies that A < B, B < C and C < D. in this problem, we will give you a set of relations of the form A < B and ask you to determine whether a sorted order has been specified or not. 

Input

Input consists of multiple problem instances. Each instance starts with a line containing two positive integers n and m. the first value indicated the number of objects to sort, where 2 <= n <= 26. The objects to be sorted will be the first n characters of the uppercase alphabet. The second value m indicates the number of relations of the form A < B which will be given in this problem instance. Next will be m lines, each containing one such relation consisting of three characters: an uppercase letter, the character "<" and a second uppercase letter. No letter will be outside the range of the first n letters of the alphabet. Values of n = m = 0 indicate end of input.

Output

For each problem instance, output consists of one line. This line should be one of the following three:

Sorted sequence determined after xxx relations: yyy...y.
Sorted sequence cannot be determined.
Inconsistency found after xxx relations.

where xxx is the number of relations processed at the time either a sorted sequence is determined or an inconsistency is found, whichever comes first, and yyy...y is the sorted, ascending sequence.

Sample Input

4 6
A<B
A<C
B<C
C<D
B<D
A<B
3 2
A<B
B<A
26 1
A<Z
0 0

Sample Output

Sorted sequence determined after 4 relations: ABCD.
Inconsistency found after 2 relations.
Sorted sequence cannot be determined.

 

Source

East Central North America 2001

 

拓扑排序：
1：找到入度为0的节点（找不到说明有环）   2：输出该节点，并删除与该节点有关的边（入度减1）   3：重复以上两步

#include <iostream>

#include <cstdio>

#include <cstring>

using namespace std;

const int MAXN=30;

bool adj[MAXN][MAXN];

int in_degree[MAXN];

char str[MAXN];

int n,m;

int topo()

{

    bool flag=true;

    memset(in_degree,0,sizeof(in_degree));

    memset(str,'',sizeof(str));

    for(int i=1;i<=n;i++)

    {

        for(int j=1;j<=n;j++)

        {

            if(adj[j])

            {

                in_degree[j]++;

            }

        }

    }

    for(int i=1;i<=n;i++)

    {

        int k=0;

        for(int j=1;j<=n;j++)

        {

            if(in_degree[j]==0)

            {

                if(k==0)

                {

                    k=j;

                }

                else

                {

                    flag=false;

                }

            }

        }

        if(k==0) return 0;

        in_degree[k]=-1;

        str[i-1]=k+'A'-1;

        for(int j=1;j<=n;j++)

        {

            if(adj[k][j])

            {

                in_degree[j]--;

            }

        }

    }

    if(flag) return 1;

    else return 2;

}

int main()

{

    int a,b,result;

    char s[4];

    while(scanf("%d%d",&n,&m)!=EOF)

    {

        if(n==0&&m==0) break;

        memset(adj,false,sizeof(adj));

        bool h=false;

        for(int i=1;i<=m;i++)

        {

            scanf("%s",s);

            a=s[0]-'A'+1;  b=s[2]-'A'+1;

            adj[a]=true;

            if(h) continue;

            result=topo();

            if(result==1)

            {

                printf("Sorted sequence determined after %d relations: %s. ",i,str);

                h=true;

            }

            if(result==0)

            {

                printf("Inconsistency found after %d relations. ",i);

                h=true;

            }

        }

        if(!h)

        printf("Sorted sequence cannot be determined. ");

    }

    return 0;

}