POJ1094--Sorting It All Out--拓补排序

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.

#include <iostream>
#include <cstdio>
using namespace std;
#define maxn 28
int c[maxn];
int G[maxn][maxn];
int topo[maxn];
int n,t;
//判断是否矛盾可直接用刘汝佳的模板。判断是否唯一确定，一个只要夹在两个中间即可
bool dfs(int u)
{
c[u]=-1;
for(int i=0;i<n;i++)
{
if(G[u][i])
{
if(c[i]==-1)
{
return false;
}
else if(c[i]==0&&!dfs(i))
{
return false;
}
}
}
c[u]=1;topo[--t]=u;
return true;
}
bool toposort()
{
t=n;
memset(c,0,sizeof(c));
for(int i=0;i<n;i++)
{
if(!c[i])
{
if(!dfs(i))
return false;
}
}
return true;
}
bool judge()
{
for(int i=1;i<n;i++)
{
if(!(G[topo[i-1]][topo[i]]))
{
return false;
}
}
return true;
}
int main()
{
int m;
while(scanf("%d%d",&n,&m)==2&&(n||m))
{
bool flag=false,maodun=false;//flag用来判几次实现序列确定。maodun用来判断是否会矛盾
memset(G,0,sizeof(G));
char a,b,c;
int sum,gg;
memset(topo,0,sizeof(topo));
for(int i=1;i<=m;i++)
{//之前我把topo的置0放到循环了。贡献了无数个WA。。
cin>>a>>b>>c;
G[a-'A'][c-'A']=1;
if(!maodun)
{
if(!toposort())
{
maodun=true;
gg=i;
}
else
{
if(!flag)
{
if(judge())
{
flag=true;
sum=i;
}
}
}
}
}
if(flag)
{
printf("Sorted sequence determined after %d relations: ",sum);
for(int i=1;i<=n;i++)
{
char xx=topo[i-1]+'A';
cout<<xx;
}
cout<<"."<<endl;
}
else if(maodun)
{
printf("Inconsistency found after %d relations.\n",gg);
}
else cout<<"Sorted sequence cannot be determined."<<endl;
}
return 0;
}