题目链接：

http://poj.org/problem?id=1094

题意：

给出一连串关系，判断3个问题：
1.有唯一解并输出
2.是否有环
3.没有唯一解

注意1，3还要输出在第几个关系可以判断出来，可知3需要判断到最后，而1可能不需要。

AC代码：

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>
#include <map>
using namespace std;

const int MAXN = 50;

map <string,int> letter;
int G[MAXN][MAXN], topo[MAXN], deep[MAXN];

int toposort(int n)
{
    int i = 1,flag = 0;
    int temp[40];
    for(int hasaki = 1; hasaki <= n; hasaki++)
    {
        temp[hasaki] = deep[hasaki];
    }
    while(i <= n)
    {
        int judge = 0;

        for(int j = 1; j <= n; j++)
        {
            if(!temp[j])
                judge++;
        }
        if(!judge)
            return 1;//有环
        else if(judge > 1)
            flag = 2;//不是唯一序
        for(int j = 1; j <= n; j++)
        {
            if(!temp[j])
            {
                temp[j]--;
                topo[i++] = j;
                for(int k = 1; k <= n; k++)
                {
                    if(G[j][k])
                    {
                        temp[k]--;
                    }
                }
                break;
            }
        }
    }
    return flag;
}

int main()
{
    int n,m;
    while(~scanf("%d %d", &n, &m))
    {
        if(!n && !m)break;
        char str[5];
        int flag = 0;
        memset(deep,0,sizeof(deep));
        memset(G,0,sizeof(G));
        for(int i = 1; i<=m; i++)
        {
            scanf("%s", &str);
            int a = str[0] - 'A' + 1, b = str[2] - 'A' + 1;
            if(flag)continue;
            G[a][b] = 1;
            deep[b]++;
            int ans = toposort(n);
            if(ans == 1)
            {
                printf("Inconsistency found after %d relations.\n", i);
                flag = 1;
            }
            else if(ans == 0)
            {
                printf("Sorted sequence determined after %d relations: ", i);
                for(int k = 1; k <= n; k++)
                    printf("%c", topo[k] - 1 + 'A');
                printf(".\n");
                flag = 1;
            }
        }
        if(!flag)
            puts("Sorted sequence cannot be determined.");
    }
}