拓扑排序 POJ 1049 Sorting It All Out

 

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.\n", 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.\n", 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 (".\n");  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 */