题目传送门
1 /*
2 拓扑排序裸题：有三种情况：
3 1. 输入时发现与之前的矛盾，Inconsistency
4 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)；
5 flag = -1 表示不确定；return 2 表示拓扑序唯一
6 3. 其他情况都是 Sorted sequence cannot be determined.
7
8 */
9 #include <cstdio>
10 #include <algorithm>
11 #include <cmath>
12 #include <iostream>
13 #include <cstring>
14 #include <string>
15 #include <queue>
16 #include <vector>
17 using namespace std;
18
19 const int MAXN = 30;
20 const int INF = 0x3f3f3f3f;
21 int in[MAXN], ans[MAXN];
22 vector<int> G[MAXN];
23 bool used[MAXN][MAXN];
24 int n, m;
25
26 int TopoSort(void)
27 {
28 int flag = 0;
29 memset (in, 0, sizeof (in));
30 for (int i=1; i<=n; ++i)
31 {
32 for (int j=0; j<G[i].size (); ++j) in[G[i][j]]++;
33 }
34
35 queue<int> Q; int cnt = 0;
36 for (int i=1; i<=n; ++i) if (!in[i]) Q.push (i);
37
38 while (!Q.empty ())
39 {
40 if (Q.size () > 1) flag = -1;
41 int u = Q.front (); Q.pop ();
42 ans[++cnt] = u;
43 for (int i=0; i<G[u].size (); ++i)
44 {
45 int v = G[u][i];
46 in[v]--;
47 if (!in[v]) Q.push (v);
48 }
49 }
50
51 if (cnt != n) return 1;
52 else if (flag == -1) return -1;
53 else return 2;
54 }
55
56 int main(void) //POJ 1049 Sorting It All Out
57 {
58 //freopen ("POJ_1094.in", "r", stdin);
59
60 char s[5];
61 while (scanf ("%d%d", &n, &m) == 2)
62 {
63 if (n == 0 && m == 0) break;
64
65 for (int i=1; i<=n; ++i) G[i].clear ();
66 memset (used, 0, sizeof (used));
67
68 int flag = 0;
69 for (int i=1; i<=m; ++i)
70 {
71 scanf ("%s", &s);
72 if (flag) continue;
73
74 int u = s[0] - 'A' + 1;
75 int v = s[2] - 'A' + 1;
76
77 if (used[v][u]) {flag = 1; printf ("Inconsistency found after %d relations.
", i); continue;}
78
79 used[u][v] = true;
80 G[u].push_back (v);
81
82 int res = TopoSort ();
83 if (res == 1) {flag = 1; printf ("Inconsistency found after %d relations.
", i); continue;}
84 else if (res == 2)
85 {
86 flag = 1;
87 printf ("Sorted sequence determined after %d relations: ", i);
88 for (int i=1; i<=n; ++i) printf ("%c", ans[i] + 'A' - 1);
89 printf (".
");
90 }
91 }
92
93 if (!flag) puts ("Sorted sequence cannot be determined.");
94 }
95
96 return 0;
97 }
98
99 /*
100 Sorted sequence determined after 4 relations: ABCD.
101 Inconsistency found after 2 relations.
102 Sorted sequence cannot be determined.
103 */