poj1094(拓扑排序)

  好吧，这个题目的题意对于英语弱渣的我来说，略难懂，实际就是，每次读入一个信息，然后判断现在是否出现有环，或者出现唯一的拓扑序列，如果直到最后一条信息读完，都没出现前面两种情况，就说没有无法决定的拓扑序列。

  wa了一发，主要原因是应该先判断是否有环，然后再来判断序列不唯一。

  思路：就是每读入一条信息，就进行一次拓扑排序，得出结果。

  

#include "stdio.h"
#include "string.h"
#include "math.h"
#include <string>
#include <queue>
#include <stack>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
using namespace std;

#define MAXM 5000
#define MAXN 26
#define max(a,b) a > b ? a : b
#define min(a,b) a < b ? a : b
#define abs(a)  a < 0 ? a : (-a)
#define Mem(a,b) memset(a,b,sizeof(a))
int Mod = 1000000007;
double pi = acos(-1.0);
double eps = 1e-6;

typedef struct{
	int f,t,w,next;
}Edge;
Edge edge[MAXM];
int head[MAXN];
int mapp[MAXN][MAXN];
int kNum;
int indree1[MAXN];
int indree2[MAXN];

int tag;
void addEdge(int f, int t, int w)
{
	edge[kNum].f = f;
	edge[kNum].t = t;
	edge[kNum].w = w;
	edge[kNum].next = head[f];
	head[f] = kNum ++;
}

int N,M;
int indree[26];

int TopSort(int n)
{
	int m = -1;
	queue<int> q;
	for(int i = 0; i < N; i ++){
		indree2[i] = indree1[i];
		if( indree2[i] == 0 )
			q.push(i);
	}
	int x = 0;
	while( !q.empty() ){
		if( q.size() > 1 )
			m = 0;	//wa在这，直接return 0了
		int t = q.front();
		q.pop();
		x ++;
		for(int k = head[t]; k != -1; k = edge[k].next ){
			indree2[edge[k].t] --;
			if( indree2[edge[k].t] == 0 )
				q.push(edge[k].t);
		}
	}
	if( x != N )
		return 1;
	if( m != -1 ) 
		return 0;
	printf("Sorted sequence determined after %d relations: ",n);
	for(int i = 0; i < N; i ++){
		indree2[i] = indree1[i];
		if( indree2[i] == 0 )
			q.push(i);
	}
	while( !q.empty() ){
		int t = q.front();
		q.pop();
		printf("%c",t + 'A');
		for(int k = head[t]; k != -1; k = edge[k].next ){
			indree2[edge[k].t] --;
			if( indree2[edge[k].t] == 0 )
				q.push(edge[k].t);
		}
	}
	printf(".\n");
	return 2;

}

void solve()
{
	tag = kNum = 0;
	Mem(head,-1);
	Mem(mapp,0);
	Mem(indree1,0);
	char s[5];

	for(int i = 0; i < M; i ++){
		getchar();
		scanf("%s",s);
		if( tag ) continue;
		int x = s[0] - 'A';
		int y = s[2] - 'A';
		indree1[y] ++;
		if( mapp[x][y] ) continue;
		addEdge(x,y,0);
		tag = TopSort(i+1);
		if( tag == 1 ){
			printf("Inconsistency found after %d relations.\n",i+1);
		}
	}
	if( tag == 0 ){
		printf("Sorted sequence cannot be determined.\n");
	}

}




int main()
{
//	freopen("d:\\test.txt", "r", stdin);

	while(cin>>N){
		cin>>M;
		if( N == 0 && M == 0 )
			break;
		solve();
	}

	return 0;
}