poj1094——Sorting It All Out（拓扑排序）

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.

题意是给出一些比较关系，求给出这些关系的过程中1 ）能否有一个确定的排序，有的话给出关系的个数并求出这个排序的序列；2）是否成环；3）不能确定
只有一个确定排序的拓扑序列的性质应该是，只有一个入度为0的节点，去掉此节点后，剩下的还是只有一个入度为0的点。按照这个性质模拟就行了。

#include <iostream>#include <cstring>#include <string>#include <vector>#include <queue>#include <cstdio>#include <set>#include <cmath>#include <algorithm>#define INF 0x3f3f3f3f#define MAXN 105#define Mod 10001using namespace std;int n,m,c;int map[30][30],temp[30],indegree[30],q[30];int topsort(int n){    int m,pos,flag=1;    c=0;    for(int i=1; i<=n; ++i)        temp[i]=indegree[i];    for(int i=1; i<=n; ++i)    {        m=0;        for(int j=1; j<=n; ++j)            if(temp[j]==0) //选择一个入度为0的点，即可能为起点            {                m++;                pos=j;            }        if(m==0)            return 0; //有环        if(m>1)            flag=-1;  //还不确定        q[c++]=pos; //该起点入栈        temp[pos]=-1;        for(int j=1;j<=n;++j)            if(map[pos][j]) //把起点去掉后，相连的点要有处理                temp[j]--;    }    return flag;}int main(){    char op[10];    while(~scanf("%d%d",&n,&m))    {        getchar();        if(n==0&&m==0)            break;        memset(indegree,0,sizeof(indegree));        memset(map,0,sizeof(map));        int s=-1,flag=0;        for(int i=1; i<=m; ++i)        {            gets(op);            if(s==0||s==1)                continue;            int x=op[0]-'A'+1;            int y=op[2]-'A'+1;            map[x][y]=1;            indegree[y]++;            s=topsort(n);            if(s==0)            {                printf("Inconsistency found after %d relations.\n",i);                flag=1;            }            if(s==1)            {                printf("Sorted sequence determined after %d relations: ",i);                for(int j=0;j<c;++j)                    printf("%c",q[j]+'A'-1);                printf(".\n");                flag=1;            }        }        if(!flag)            printf("Sorted sequence cannot be determined.\n");    }    return 0;}