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.
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.
解题思路：拓扑排序裸题，边读边进行拓扑排序，如果出队个数小于n，说明有环，返回0，输出此时的位置，下次直接跳过所有操作即可，直接循环读取字符串；如果返回1，则次序唯一，直接输出次序即可，同样下次直接跳过其他操作；如果前两者都没发生，最后才可以确定次序不唯一，即队中度为0的元素大于1这种情况。
AC代码：
1 #include<iostream> 2 #include<vector> 3 #include<queue> 4 #include<string.h> 5 #include<map> 6 #include<cstdio> 7 using namespace std; 8 const int maxn=26; 9 int n,m,out[maxn],InDeg[maxn];char cs[5]; 10 vector<int> vec[maxn]; 11 queue<int> que; 12 int topsort(){//flag:-1次序不确定；1：次序唯一 13 while(!que.empty())que.pop();//清空 14 int cnt=0,flag=1,in[maxn]; 15 for(int i=0;i<n;++i)in[i]=InDeg[i];//拷贝每个节点的入度数 16 for(int i=0;i<n;++i) 17 if(!in[i])que.push(i);//先将入度为0的进队 18 while(!que.empty()){ 19 if(que.size()>1)flag=-1;//次序不确定 20 int now=que.front();out[cnt++]=now;que.pop(); 21 for(size_t i=0;i<vec[now].size();++i) 22 if(--in[vec[now][i]]==0)que.push(vec[now][i]); 23 } 24 if(cnt!=n)return 0;//成环 25 return flag; 26 } 27 int main(){ 28 while(cin>>n>>m&&(n+m)){ 29 getchar(); 30 memset(InDeg,0,sizeof(InDeg)); 31 for(int i=0;i<n;++i)vec[i].clear();//全部清空 32 int sign=0; 33 for(int i=1;i<=m;++i){ 34 cin>>cs; 35 if(sign)continue;//跳过下面的操作，表示结论已出 36 int f1=cs[0]-'A',f2=cs[2]-'A'; 37 vec[f1].push_back(f2); 38 InDeg[f2]++; 39 int k=topsort(); 40 if(k==0){//成环 41 printf("Inconsistency found after %d relations.\n",i); 42 sign=1;//同时标记为1 43 } 44 if(k==1){//有序 45 printf("Sorted sequence determined after %d relations: ",i); 46 for(int j=0;j<n;++j)printf("%c",out[j]+'A'); 47 printf(".\n"); 48 sign=1; 49 } 50 } 51 if(!sign)printf("Sorted sequence cannot be determined.\n");//最后才确定次序是否唯一 52 } 53 return 0; 54 }