nyoj Sorting It All Out (拓扑排序)

三种情况分别是：

1. 在某位置可以确定拓扑排序。

2. 在某位置出现了环

3. 到最后都不能确定拓扑排序(某一位置入度为0的点有多个)，可以续输入执行下去。

每输入一组数据都要做一次判断

#include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
using namespace std;
const int N = 105;
int n, m, in[N], temp[N], ans[N], t, pos, num;
char X, O, Y;
vector<int>G[N];
queue<int>q;

void init(){
    memset(in, 0, sizeof(in));
    for (int i = 0; i <= n; ++i){
        G[i].clear();
    }
}

int topoSort(){
    while (!q.empty())q.pop();
    for (int i = 0; i<n; ++i)if (in[i] == 0){
        q.push(i);
    }
    pos = 0;
    bool unSure = false;
    while (!q.empty()){
        if (q.size()>1) unSure = true;
        int t = q.front();
        q.pop();
        ans[pos++] = t;
        for (int i = 0; i<G[t].size(); ++i){
            if (--in[G[t][i]] == 0)
                q.push(G[t][i]);
        }
    }
    if (pos<n) return 1;
    if (unSure)  return 2;
    return 3;
}

int main(){
    int x, y, i, flag, ok, stop;
    while (~scanf("%d%d%*c", &n, &m),n+m){
        init();
        flag = 2;
        ok = false;
        for (i = 1; i <= m; ++i){
            scanf("%c%c%c%*c", &X, &O, &Y);
            if (ok) continue; 
            x = X - 'A', y = Y - 'A';
            if (O == '<'){
                G[y].push_back(x);
                ++in[x];
            }
            else if (O == '>'){
                G[x].push_back(y);
                ++in[y];
            }
            memcpy(temp, in, sizeof(in));
            flag = topoSort();
            memcpy(in, temp, sizeof(temp));
            if (flag != 2){
                stop = i;
                ok = true;
            }
        }
        if (flag == 3){
            printf("Sorted sequence determined after %d relations: ", stop);
            for (int i = pos - 1; i >= 0; --i)
                printf("%c", ans[i] + 'A');
            printf(".
");
        }
        else if (flag == 1){
            printf("Inconsistency found after %d relations.
", stop);
        }
        else{
            printf("Sorted sequence cannot be determined.
");
        }
    }
    return 0;
}