Problem: 207. 课程表

思路

👨‍🏫 三叶题解

复杂度

时间复杂度: $O(n+m)$
空间复杂度: $O(n+m)$

Code

class Solution{
	int N = 100010, M = 5010, idx;
	int[] in = new int[N];// in[i] 表示节点 i 的入度
	int[] h = new int[N];
	int[] e = new int[N];
	int[] ne = new int[N];

//	添加一条边 a --> b
	void add(int a, int b)
	{
		e[idx] = b;
		ne[idx] = h[a];
		h[a] = idx++;
		in[b]++;
	}

	public boolean canFinish(int n, int[][] g)
	{
		Arrays.fill(h, -1);//初始化头结点
		for (int[] a : g)
			add(a[1], a[0]);
		int ans = 0;
		Queue<Integer> q = new LinkedList<>();
		for (int i = 0; i < n; i++)
			if (in[i] == 0)
				q.add(i);
		while (!q.isEmpty())
		{
			int x = q.poll();
			ans++;
			for (int i = h[x]; i != -1; i = ne[i])
			{
				int j = e[i];
				in[j]--;
				if (in[j] == 0)
					q.add(j);
			}
		}
		return ans == n;
	}
}