【题解】poj1094 Sorting It All Out 拓扑排序

题目链接

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

Sample Output

Sorted sequence determined after 4 relations: ABCD.
Inconsistency found after 2 relations.
Sorted sequence cannot be determined.

---

李煜东书上讲的是floyd进行传递闭包，我直接写的拓扑排序。
结果写死了……折腾半天，去学了大佬题解的代码实现。

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
int n,m;
bool check;
int d[30][30],deg[30],in[30],q[30];
char sor[30];
string s;
int toposort()
{
    int flag=1,c=0;
    memcpy(in,deg,sizeof(deg));
    int cnt=0,tp;
    for(int i=1;i<=n;i++)
    {
        cnt=tp=0;
        for(int j=1;j<=n;j++)
            if(!in[j])
            {
                cnt++;tp=j;
            }
        if(!cnt)return 0;
        if(cnt>1)flag=-1;
        q[c++]=tp;in[tp]=-1;
        for(int j=1;j<=n;j++)if(d[tp][j]==1)in[j]--;
    }
    return flag;
}
int main()
{
    //freopen("in.txt","r",stdin);
    std::ios::sync_with_stdio(false);
    while(cin>>n>>m)
    {
        if(n==0&&m==0)break;
        check=false;
        memset(d,0,sizeof(d));
        memset(deg,0,sizeof(deg));
        for(int i=1;i<=m;i++)
        {
            cin>>s;
            if(check)continue;
            int u=s[0]-'A'+1,v=s[2]-'A'+1;
            d[u][v]=1;deg[v]++;
            int state=toposort();
            if(state==0)
            {
                printf("Inconsistency found after %d relations.\n",i);
                check=true;
            }
            else if(state==1)
            {
                printf("Sorted sequence determined after %d relations: ",i);
                check=true;
                for(int j=0;j<n;j++)printf("%c",q[j]+'A'-1);
                printf(".\n");
            }
        }
        if(!check)printf("Sorted sequence cannot be determined.\n");
    }
    return 0;
}

总结

拓扑排序判断是否有环等等……我基本功还是不扎实。