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 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

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.\n",flag2,ans);
85          continue;
86       }
87       if(flag1)
88       {
89          printf("Inconsistency found after %d relations.\n",flag1);
90          continue;
91       }
92       printf("Sorted sequence cannot be determined.\n");
93     }
94     return 0;
95 }