拓扑排序 Codeforces Round #290 (Div. 2) C. Fox And Names

题目传送门

 /*
     给出n个字符串，求是否有一个“字典序”使得n个字符串是从小到大排序
     拓扑排序
     详细解释：http://www.2cto.com/kf/201502/374966.html
 */
 #include <cstdio>
 #include <iostream>
 #include <cstring>
 #include <string>
 #include <map>
 #include <algorithm>
 #include <vector>
 #include <set>
 #include <queue>
 #include <cmath>
 using namespace std;

 const int MAXN = 1e6 + ;
 const int INF = 0x3f3f3f3f;
 char s[][];
 int in[];
 bool lin[][];
 char ans[];

 bool TopoSort(void)
 {
     queue<int> q;
     int cnt = ;
     for (int i=; i<=; ++i)
     {
         if (in[i] == )
         {
             q.push (i);
             ans[cnt++] = 'a' + i - ;
         }
     }
     while (!q.empty ())
     {
         int now = q.front ();   q.pop ();
         for (int i=; i<=; ++i)
         {
             if (lin[now][i])
             {
                 in[i]--;
                 if (in[i] == )
                 {
                     q.push (i);
                     ans[cnt++] = 'a' + i - ;
                 }
             }
         }
     }
     ans[] = '\0';
     if (cnt < )   return false;
     else    return true;
 }

 int main(void)
 {
     //freopen ("C.in", "r", stdin);

     int n;

     while (~scanf ("%d", &n))
     {
         memset (lin, false, sizeof (lin));
         memset (in, , sizeof (in));
         for (int i=; i<=n; ++i)
         {
             scanf ("%s", &s[i]);
         }

         bool flag = true;
         for (int i=; i<=n- && flag; ++i)
         {
             int len1 = strlen (s[i]);
             int len2 = strlen (s[i+]);
             bool ok = false;
             for (int j=; j<=len1- && j<=len2- && !ok; ++j)
             {
                 if (s[i][j] != s[i+][j])
                 {
                     ok = true;
                     if (!lin[s[i][j]-'a'+][s[i+][j]-'a'+])
                     {
                         in[s[i+][j]-'a'+]++;
                         lin[s[i][j]-'a'+][s[i+][j]-'a'+] = true;
                     }
                 }
             }
             if (!ok && len1 > len2) flag = false;
         }

         if (!flag)  puts ("Impossible");
         else
         {
             flag = TopoSort ();
             if (!flag)   puts ("Impossible");
             else    printf ("%s\n", ans);
         }
     }

     return ;
 }