拓扑排序（栈）——POJ 1094
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Sorting It All Out
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Time Limit:1000MS Memory Limit:10000KB 64bit IO Format:%I64d & %I64u
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.
#include <cstdio>
#include <cstring>
#include <queue>
#include <iostream>
using namespace std;
const int INF=1<<30;
const int MAXN=7500+10;
int n,m;
int G[30][30];
int in[30];
int out[30];
int in1[30];
int topo[30];
int countt;
int toposort()
{
memset(topo,-1,sizeof(topo));
memset(in1,0,sizeof(in1));
memcpy(in1,in,sizeof(in));
queue<int>q;
int t=0,qin=0;
for(int u=0; u<n; u++){
if(!in[u] && out[u]) q.push(u);
if(in[u] || out[u]) t++;
}
countt=0;
while(!q.empty())
{
if(q.size()>1) qin=1;
int u=q.front();
q.pop();
topo[countt++]=u;
for(int v=0; v<n; v++){
if(G[u][v]) in1[v]--;
if(G[u][v] && !in1[v]) q.push(v);
}
}
if(countt!=t) return 0;//有环
if(t!=n) return 1;//边没有连完
if(t==n && qin) return 2;//大小不确定
if(t==n && !qin) return 3;//大小确定
}
int main()
{
//freopen("in.txt","r",stdin);
while(cin>>n>>m, n+m)
{
memset(G,0,sizeof(G));
memset(in,0,sizeof(in));
int i,j,ok=-1;
int num1,num2;
for(i=0; i<m; i++){
char ch1,ch2,ch3;
cin>>ch1>>ch2>>ch3;
//cout<<ch1<<" "<<ch3<<endl;
int a=ch1-'A';
int b=ch3-'A';
G[a][b]=1;
in[b]++;
out[a]++;
if(ok==0 || ok==3) continue;
ok=toposort();
if(ok==0) num1=i+1;
if(ok==3) num2=i+1;
}
if(!ok) cout<<"Inconsistency found after "<<num1<<" relations."<<endl;
else if(ok==3) {
cout<<"Sorted sequence determined after "<<num2<<" relations: ";
for(i=0; i<countt; i++)
cout<<char(topo[i]+'A');
cout<<"."<<endl;
}
else cout<<"Sorted sequence cannot be determined."<<endl;
}
return 0;
}
bool dfs(int u)
{
c[c]=-1;
for(int v=0; v<n; v++){
if(G[u][v]){
if(c[v]<0) return false;//存在有向环，退出
else if(!c[v] && !dfs(v)) return false;//后面的节点存在有向环，退出；
}
}
c[u]=1; topo[--t]=u;
return true;
}
bool toposort()
{
t=n;
memset(c,0,sizeof(c));
for(int u=0; u<n; u++)
{
if(!c[u]){
if(!dfs(u)) return false;
}
}
return true;
}