题目传送门
1 /*
2 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
3 拓扑排序
4 详细解释：http://www.2cto.com/kf/201502/374966.html
5 */
6 #include <cstdio>
7 #include <iostream>
8 #include <cstring>
9 #include <string>
10 #include <map>
11 #include <algorithm>
12 #include <vector>
13 #include <set>
14 #include <queue>
15 #include <cmath>
16 using namespace std;
17
18 const int MAXN = 1e6 + 10;
19 const int INF = 0x3f3f3f3f;
20 char s[110][110];
21 int in[110];
22 bool lin[27][27];
23 char ans[27];
24
25 bool TopoSort(void)
26 {
27 queue<int> q;
28 int cnt = 0;
29 for (int i=1; i<=26; ++i)
30 {
31 if (in[i] == 0)
32 {
33 q.push (i);
34 ans[cnt++] = 'a' + i - 1;
35 }
36 }
37 while (!q.empty ())
38 {
39 int now = q.front (); q.pop ();
40 for (int i=1; i<=26; ++i)
41 {
42 if (lin[now][i])
43 {
44 in[i]--;
45 if (in[i] == 0)
46 {
47 q.push (i);
48 ans[cnt++] = 'a' + i - 1;
49 }
50 }
51 }
52 }
53 ans[26] = ' ';
54 if (cnt < 26) return false;
55 else return true;
56 }
57
58 int main(void)
59 {
60 //freopen ("C.in", "r", stdin);
61
62 int n;
63
64 while (~scanf ("%d", &n))
65 {
66 memset (lin, false, sizeof (lin));
67 memset (in, 0, sizeof (in));
68 for (int i=1; i<=n; ++i)
69 {
70 scanf ("%s", &s[i]);
71 }
72
73 bool flag = true;
74 for (int i=1; i<=n-1 && flag; ++i)
75 {
76 int len1 = strlen (s[i]);
77 int len2 = strlen (s[i+1]);
78 bool ok = false;
79 for (int j=0; j<=len1-1 && j<=len2-1 && !ok; ++j)
80 {
81 if (s[i][j] != s[i+1][j])
82 {
83 ok = true;
84 if (!lin[s[i][j]-'a'+1][s[i+1][j]-'a'+1])
85 {
86 in[s[i+1][j]-'a'+1]++;
87 lin[s[i][j]-'a'+1][s[i+1][j]-'a'+1] = true;
88 }
89 }
90 }
91 if (!ok && len1 > len2) flag = false;
92 }
93
94 if (!flag) puts ("Impossible");
95 else
96 {
97 flag = TopoSort ();
98 if (!flag) puts ("Impossible");
99 else printf ("%s
", ans);
100 }
101 }
102
103 return 0;
104 }