SDNU 1089.拓扑排序（拓扑判环小顶堆）

#include<bits/stdc++.h>
using namespace std;

int n, m, a, b, d[1000+8], num, cnt;
vector<int>t[1000+8];
queue<int>q;
int ans[1000+8], sign[1000+8];
bool flag = 0;
priority_queue<int, vector<int>, greater<int> >you;

void bfs()
{
    while(!you.empty())
    {
        int f = you.top();
        you.pop();
        if(flag)printf(" ");
        flag = 1;
        printf("%d", f);
        for(int i = 0; i<t[f].size(); i++)
        {
            d[t[f][i]]--;
            if(d[t[f][i]] == 0 && !sign[t[f][i]])
            {
                you.push(t[f][i]);
                sign[t[f][i]] = 1;
            }
//            }
        }
    }
}

int get(int miao)//判环
{
    int in[1000+8],si[1000+8];
    for(int i = 0; i <= miao; i++)
    {
        in[i] = d[i];
        si[i] = sign[i];
    }
    num = miao;
    while(!q.empty())
    {
        int f = q.front();
        num--;
        q.pop();
        for(int i = 0; i<t[f].size(); i++)
        {
            in[t[f][i]]--;
            if(!in[t[f][i]] && !si[t[f][i]])
            {
                q.push(t[f][i]);
                si[t[f][i]] = 1;
            }
        }
    }
    return num;
}

int main()
{
    while(~scanf("%d%d", &n, &m) && (n+m))
    {
        memset(d, 0, sizeof(d));
        memset(sign, 0, sizeof(sign));
        for(int i = 0; i<1000+8; i++)t[i].clear();
        while(!q.empty())q.pop();
        for(int i = 0; i<m; i++)
        {
            scanf("%d%d", &a, &b);
            t[a].push_back(b);//存图
            d[b]++;//入度++
        }
        for(int i = 1; i <= n; i++)
        {
            if(!d[i] && !sign[i])//查找入度为0的节点，再插入队列，并标记它用过了
            {
                q.push(i);
                you.push(i);
                sign[i] = 1;
            }
        }
        if(get(n) == 0)//如果不存在环
        {
            bfs();//用bfs输出结果
        }
        else printf("IMPOSABLE\n");
    }
    return 0;
}