拓扑排序 POJ 1049 Sorting It All Out

题目传送门

 /*
     拓扑排序裸题：有三种情况：
                     1. 输入时发现与之前的矛盾，Inconsistency
                     2. 拓扑排序后，没有n个点(先判断cnt，即使一些点没有边连通，也应该是n，此时错误是有环)；
                         flag = -1 表示不确定；return 2 表示拓扑序唯一
                     3.     其他情况都是 Sorted sequence cannot be determined.

 */
 #include <cstdio>
 #include <algorithm>
 #include <cmath>
 #include <iostream>
 #include <cstring>
 #include <string>
 #include <queue>
 #include <vector>
 using namespace std;

 const int MAXN = ;
 const int INF = 0x3f3f3f3f;
 int in[MAXN], ans[MAXN];
 vector<int> G[MAXN];
 bool used[MAXN][MAXN];
 int n, m;

 int TopoSort(void)
 {
     int flag = ;
     memset (in, , sizeof (in));
     for (int i=; i<=n; ++i)
     {
         for (int j=; j<G[i].size (); ++j)    in[G[i][j]]++;
     }

     queue<int> Q;    int cnt = ;
     for (int i=; i<=n; ++i)    if (!in[i])    Q.push (i);

     while (!Q.empty ())
     {
         if (Q.size () > )    flag = -;
         int u = Q.front ();        Q.pop ();
         ans[++cnt] = u;
         for (int i=; i<G[u].size (); ++i)
         {
             int v = G[u][i];
             in[v]--;
             if (!in[v])    Q.push (v);
         }
     }

     if (cnt != n)    return ;
     else if (flag == -)    return -;
     else    return ;
 }

 int main(void)        //POJ 1049 Sorting It All Out
 {
     //freopen ("POJ_1094.in", "r", stdin);

     char s[];
     while (scanf ("%d%d", &n, &m) == )
     {
         if (n ==  && m == )    break;

         for (int i=; i<=n; ++i)    G[i].clear ();
         memset (used, , sizeof (used));

         int flag = ;
         for (int i=; i<=m; ++i)
         {
             scanf ("%s", &s);
             if (flag)    continue;

             int u = s[] - 'A' + ;
             int v = s[] - 'A' + ;

             if (used[v][u])        {flag = ;    printf ("Inconsistency found after %d relations.\n", i);    continue;}

             used[u][v] = true;
             G[u].push_back (v);

             int res = TopoSort ();
             if (res == )    {flag = ;    printf ("Inconsistency found after %d relations.\n", i);    continue;}
             else if (res == )
             {
                 flag = ;
                 printf ("Sorted sequence determined after %d relations: ", i);
                 for (int i=; i<=n; ++i)    printf ("%c", ans[i] + 'A' - );
                 printf (".\n");
             }
         }

         if (!flag)    puts ("Sorted sequence cannot be determined.");
     }

     return ;
 }

 /*
 Sorted sequence determined after 4 relations: ABCD.
 Inconsistency found after 2 relations.
 Sorted sequence cannot be determined.
 */