poj1094

题目大意：

     给出多组字母之间大小关系，判断是否存在一个绝对的顺序排列

解题思路：

这依然是道拓扑排序的题目，需要注意因为题目要求输出判断出具体顺序的条件序号，所以我们每读入一个条件就需要进行一次拓扑排序

还需注意拓扑排序时需要注意是否有环

#include <iostream>#include <stdio.h>#include <stdlib.h>#include <string.h>using namespace std;#define alph 26int N,M;int graph[alph][alph] = {};int degree[alph] = {};int degreeTmp[alph] = {};int result[alph] = {};//-----------------------------// 拓扑排序// // 每次寻找一个入度为0的点，加入结果队列// 如果某次包括多个入度为0的点则表示还不能确定顺序// 但还是要继续下去，因为要判断是否包含环//-----------------------------int TopologicalSorting(){int j = 0;int count;int flag;int returnValue = 0;memset(result,0,sizeof(result));memcpy(degreeTmp,degree,sizeof(degree));for(int j=0;j<N;j++){count = 0;flag = 0;for(int i=0;i<N;i++)if(degreeTmp[i] == 0){count++;flag = i;}if(count == 0)//有环return 2;else if(count > 1)// 没绝对顺序returnValue = 1;result[j] = flag;// 修改degreedegreeTmp[flag] = -1;for(int i=0;i<N;i++)if(graph[flag][i] == 1)degreeTmp[i]--;}return returnValue;}int main(){while(cin >> N >> M&& (N | M) ){memset(graph,0,sizeof(graph));memset(degree,0,sizeof(degree));bool toContinue = false;for(int i = 0;i < M;i++){char in,out,tmp;cin >> in >> tmp >> out;if(toContinue)continue;// 更新图信息graph[(int)(out-'A')][(int)(in-'A')] = 1;degree[(int)(in-'A')]++;int returnValue = TopologicalSorting();if(returnValue == 0)//找到顺序{cout << "Sorted sequence determined after "<<i+1<<" relations: ";for(int j=N-1;j>=0;j--)cout <<(char)(result[j]+'A');cout << "."<<endl;toContinue = true;}else if(returnValue == 1)// 没找到绝对顺序{// do nothing}else// 有环{cout <<"Inconsistency found after "<<i+1<<" relations."<<endl;toContinue = true;}}if(!toContinue)cout << "Sorted sequence cannot be determined."<<endl;}return 0;}