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] = '\0'; 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\n", ans); 100 } 101 } 102 103 return 0; 104 }
