【BestCoder Round #25】A(拓扑排序，找环）

题意不难看出是一道简单拓扑题目，自己对于拓扑一直没能掌握。这边就不写思路了，直接上代码了。

我的代码：

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
bool DIS[110][110];
int r[110];
bool vis[110];
int main()
{
	int n, m;
	while (cin >> n >> m)
	{
		int a, b;
		memset(r, 0, sizeof(r));
		memset(DIS, 0, sizeof(DIS));
		memset(vis, 0, sizeof(vis));
		for (int i = 0; i < m; i++)
		{
			scanf("%d%d", &b, &a);
			if (DIS[a][b] == 0)
			{
				DIS[a][b] = 1;
				r[b]++;
			}
		}
		if (n == 1)
		{
			cout << "NO" << endl;
			continue;
		}
		int counts = 0;
		while (counts < n)
		{
			int mid=-1;
			int i;
			for (i = 1; i <=n;i++)
			if (r[i] == 0&&!vis[i])
			{
				mid = i;
				vis[i] = 1;
				break;
			}
			if (mid==-1)
				break;
			for (int i = 1; i <= n;i++)
			if (DIS[mid][i])
				r[i]--;
			counts++;
		}
		if (counts == n)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
}

别人的代码（比较规范的拓扑）：

#include<iostream>
#include<cstdio>
#include<vector>
#include<cstring>
#include<queue>
using namespace std;
int main()
{
	int r[110];
	int n, m;
	while (cin >> n >> m)
	{
		int a, b;
		vector<int>g[110];
		memset(r, 0, sizeof(r));
		for (int i = 0; i < m; i++)
		{
			scanf("%d%d", &a, &b);
			g[b].push_back(a);
			r[a]++;
		}
		queue<int> q;
		for (int i = 1; i <= n;i++)
		if (r[i] == 0)
			q.push(i);
		int counts = 0;
		while (!q.empty())
		{
			int k = q.front();
			q.pop();
			for (int i = 0; i < g[k].size(); i++)
			{
				int v = g[k][i];
				r[v]--;
				if (r[v] == 0)
					q.push(v);
			}
			counts++;
		}
		cout << (counts == n ? "YES":"NO")<< endl;
	}
}