1.链接地址：
http://poj.org/problem?id=1094
http://bailian.openjudge.cn/practice/1094
2.题目：
Sorting It All Out
Time Limit: 1000MS Memory Limit: 10000K Total Submissions: 25547 Accepted: 8861 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 0Sample Output
Sorted sequence determined after 4 relations: ABCD. Inconsistency found after 2 relations. Sorted sequence cannot be determined.Source
3.思路：
4.代码：
1 #include <iostream> 2 #include <cstring> 3 #include <cstdio> 4 #include <cstdlib> 5 6 using namespace std; 7 8 int n,m; 9 int map[30][30]; 10 int reg[100]; 11 int in[30],out[30]; 12 char ans[30]; 13 int stack[30]; 14 void fun() 15 { 16 int i,j; 17 for(i='A',j=1;i<='Z';i++,j++)reg[i]=j; 18 } 19 void toposort(char *ans) 20 { 21 int i,top=0,u,s=0; 22 for(i=1;i<=n;i++) 23 if(in[i]==0)stack[top++]=i; 24 while(top!=0) 25 { 26 u=stack[--top]; 27 ans[s++]=u+64; 28 for(i=1;i<=n;i++) 29 { 30 if(map[u][i]) 31 { 32 in[i]--; 33 if(!in[i])stack[top++]=i; 34 } 35 } 36 } 37 ans[s]=0; 38 } 39 int main() 40 { 41 int i,j,x,y,k,flag1,flag2,flag; 42 fun(); 43 char ch[5]; 44 while(1) 45 { 46 flag1=flag2=0; 47 memset(map,0,sizeof(map)); 48 scanf("%d%d",&n,&m); 49 if(n==0&&m==0)break; 50 for(i=1;i<=m;i++) 51 { 52 flag=1; 53 scanf("%s",ch); 54 x=reg[ch[0]]; 55 y=reg[ch[2]]; 56 map[x][y]=1; 57 if(x==y)flag1=i; 58 memset(in,0,sizeof(in)); 59 memset(out,0,sizeof(out)); 60 if(!flag1&&!flag2) 61 for(j=1;j<=n;j++) 62 for(k=1;k<=n;k++) 63 { 64 if(j!=x&&k!=y)map[j][k]=map[j][k]||(map[j][x]&&map[y][k]); 65 if(j==x&&k!=y)map[j][k]=map[j][k]||map[y][k]; 66 if(j!=x&&k==y)map[j][k]=map[j][k]||map[j][x]; 67 if(map[j][k]) 68 { 69 out[j]++; 70 in[k]++; 71 } 72 } 73 j=1; 74 if(!flag1) 75 for(j=1;j<=n;j++) 76 { 77 if(map[j][j])flag1=i; 78 if(in[j]+out[j]!=n-1)flag=0; 79 } 80 if(flag&&!flag2&&j>n){flag2=i;toposort(ans);} 81 } 82 if(flag2) 83 { 84 printf("Sorted sequence determined after %d relations: %s. ",flag2,ans); 85 continue; 86 } 87 if(flag1) 88 { 89 printf("Inconsistency found after %d relations. ",flag1); 90 continue; 91 } 92 printf("Sorted sequence cannot be determined. "); 93 } 94 return 0; 95 }