ACM篇：POJ 1094 -- Sorting It All Out

拓撲排序。

每次輸入一個關係就進行一次拓撲排序，若得出結果則以後不再進行。
若找不到入度爲零的點且存在未刪除的點，則一定有環。
若入度爲零的點超過一個，則無序，但還需繼續判斷是否矛盾。

注意：
矛盾的優先級大於無序。
degree 在topsort需要用臨時數組。

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <vector>
#include <cstring>
using namespace std;

const int MAX = 26;

int n, m;
int indeg[MAX+1];
bool joint[MAX+1][MAX+1];
vector<int> road[MAX+1];
vector<int> zero_indeg;
vector<int> sequence;

int readchar()
{
    int t;
    while (t = getchar())
    {
        if (isalpha(t))
            break;
    }
    return t;
}

void _delete(int x, int *temp_indeg)
{
    for (int i = 0; i < road[x].size(); i++)
        temp_indeg[road[x][i]]--;
    temp_indeg[x] = -1;
}

int top_sort()
{
    sequence.clear();
    int temp_indeg[MAX+1];
    for (int i = 1; i <= MAX; i++)
        temp_indeg[i] = indeg[i];

    int flag = 1;
    while (sequence.size() < n)
    {   
        zero_indeg.clear();
        for (int i = 1; i <= n; i++)
        {
            if (!temp_indeg[i]) zero_indeg.push_back(i);
        }

        int sz = zero_indeg.size();

        if (!sz) return -1;             // circle
        else 
        {
            if (sz > 1) flag = 0;
            for (int i = 0; i < sz; i++)
            {
                sequence.push_back(zero_indeg[i]);
                _delete(zero_indeg[i], temp_indeg);
            }
        }       
    }
    return flag;
}
int main()
{
    int sign;
    int cnt;
    while (scanf("%d%d", &n, &m) && (n || m))
    {
        char a;
        char b;

        sign = 0;
        cnt = 0;
        memset(indeg, 0, sizeof(indeg));
        memset(joint, 0, sizeof(joint));
        for (int i = 1; i <= MAX; i++)
            road[i].clear();
        for (int i = 1; i <= m; i++)
        {
            a = readchar() - 'A' + 1;
            getchar();
            b = getchar() - 'A' + 1;
            if (!joint[a][b])
            {
                joint[a][b] = true;
                road[a].push_back(b);
                indeg[b]++;

                if(!sign) 
                {
                    sign = top_sort();
                    cnt++;
                }   
            }
        }

        if (sign == 1) 
        {
            printf("Sorted sequence determined after %d relations: ", cnt);
            for (int i = 0; i < n; i++)
                printf("%c", sequence[i] - 1 +'A');
            printf(".\n");
        }
        else if (sign == -1)
        {
            printf("Inconsistency found after %d relations.\n", cnt);
        }
        else if (!sign)
        {
            printf("Sorted sequence cannot be determined.\n");
        }
    }
    return 0;
}