POJ1094 拓扑排序
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Sorting It All Out
Time Limit: 1000MS Memory Limit: 10000K
Total Submissions: 31001 Accepted: 10766
Description
An ascending sorted sequence of distinct values is one in which some form of a less-than operator is used to order the elements from smallest to largest. For example, the sorted sequence A, B, C, D implies that A < B, B < C and C < D. in this problem, we will give you a set of relations of the form A < B and ask you to determine whether a sorted order has been specified or not.
Input
Input consists of multiple problem instances. Each instance starts with a line containing two positive integers n and m. the first value indicated the number of objects to sort, where 2 <= n <= 26. The objects to be sorted will be the first n characters of the uppercase alphabet. The second value m indicates the number of relations of the form A < B which will be given in this problem instance. Next will be m lines, each containing one such relation consisting of three characters: an uppercase letter, the character “<” and a second uppercase letter. No letter will be outside the range of the first n letters of the alphabet. Values of n = m = 0 indicate end of input.
Output
For each problem instance, output consists of one line. This line should be one of the following three:
Sorted sequence determined after xxx relations: yyy…y.
Sorted sequence cannot be determined.
Inconsistency found after xxx relations.
where xxx is the number of relations processed at the time either a sorted sequence is determined or an inconsistency is found, whichever comes first, and yyy…y is the sorted, ascending sequence.
Sample Input
4 6
A < B
A < C
B < C
C < D
B < D
A < B
3 2
A < B
B < A
26 1
A < Z
0 0
Sample Output
Sorted sequence determined after 4 relations: ABCD.
Inconsistency found after 2 relations.
Sorted sequence cannot be determined.
题意:
给出一个图里字母的先后顺序,若能进行拓扑排序,则在那一步输出排序结果,如果成环,则无法排序。如果没有用到所以字母,也是不能排序。
题解:
拓扑很好写,用个数组记录入度即可。问题是这个题的分类。它是只要在过程中能排序排序好了就可以输出了。成环也可以直接输出了。
#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <map>#include <queue>#include <vector>#define f(i,a,b) for(i = a;i<=b;i++)#define fi(i,a,b) for(i = a;i>=b;i--)using namespace std;int InE[30];int G[30][30];char ss[30];int n,m,Ti,S;int Topo(){ int cnt = 0; int tem[30] = {0},fla = 3; int i,j,S; f(i,1,n) tem[i] = InE[i]; f(i,1,n){ int sum = 0; f(j,1,n) if(!tem[j]){sum++;S = j;} if(sum == 0) return 1; if(sum >1) fla = 2; ss[cnt++] = 'A' + S - 1; tem[S] = -1; f(j,1,n) if(G[S][j]) tem[j]--; } ss[n] = '\0'; return fla;}int main(){ while(~scanf("%d%d",&n,&m)&&(n||m)){ getchar(); char a,b; int i,j,flag = 0,ok = 0; memset(G,0,sizeof(G)); memset(InE,0,sizeof(InE)); f(i,1,m){ scanf("%c<%c",&a,&b); getchar(); if(ok) continue; G[a-'A'+1][b-'A'+1] = 1; InE[b-'A'+1]++; flag = Topo(); if(1 == flag){ printf("Inconsistency found after %d relations.\n",i); ok = 1; } else if(3 == flag){ printf("Sorted sequence determined after %d relations: %s.\n",i,ss); ok = 1; } } if(!ok) printf("Sorted sequence cannot be determined.\n"); } return 0;}
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