每次加入一个关系都要进行拓扑排序，不过在排序过程中需要判断是否出现多个入度为0的点，如果出现了就说明不能确定大小关系。不论出不出现多个入度为0的点拓扑排序都要进行到最后来判断是否出现环，因为一旦出现环就不用再比了，一定是不能排序的。
另外，注意输出序列之后的那个逗号。。。。。。
Sorting It All Out
Time Limit : 2000/1000ms (Java/Other) Memory Limit : 20000/10000K (Java/Other)
Total Submission(s) : 24 Accepted Submission(s) : 13
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
PKU
#include <iostream>
#include <cstring>
#include <stdio.h>
#define inf 0xfffffff
#define maxn 27 using namespace std; int n,m;
char les,first,second;
int mp[maxn][maxn],ans[maxn]; int topo()
{
int i,j,res=0;
int into[maxn]={0};
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
if(mp[i][j]>0)
into[j]++;
} }
for(i=0;i<n;i++)
{
int cnt=0,tmp=-1;
for(j=0;j<n;j++)
{
if(into[j]==0)
{
cnt++;
tmp=j;
}
}
if(cnt==0)
return 1;
if(cnt>1)
res=2;
ans[i]=tmp;
into[tmp]=-1;
for(j=0;j<n;j++)
{
if(mp[tmp][j]>0)
into[j]--;
}
}
if(res==2)
return 2;
else
return 3;
} int main()
{
// freopen("in.txt","r",stdin);
while(cin>>n>>m&&(n+m))
{
memset(mp,0,sizeof(mp));
memset(ans,0,sizeof(ans));
int i,tp=2,last=0,num=0;
for(i=0;i<m;i++)
{
cin>>first>>les>>second;
int a=first-'A',b=second-'A';
mp[a][b]=1;
if(tp==2)
{
tp=topo();
}
if(last==2&&(tp==1||tp==3))
{
num=i+1;
}
last=tp;
}
if(tp==3)
{
cout<<"Sorted sequence determined after "<<num<<" relations: ";
for(i=0;i<n;i++)
cout<<char(ans[i]+'A');
cout<<'.';
cout<<endl;
}
else if(tp==2)
cout<<"Sorted sequence cannot be determined."<<endl;
else if(tp==1)
cout<<"Inconsistency found after "<<num<<" relations."<<endl; }
return 0;
}