207. 课程表 & 210. 课程表 II (拓扑排序)

207. 课程表

拓扑排序的BFS实现

class Solution {	
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        if (numCourses <= 0) {
            return false;
        }
        // 特判
        if (0 == prerequisites.length) {
            return true;
        }
    	int[] indegrees = new int[numCourses]; //入度数组
        HashSet<Integer>[] adj = new HashSet[numCourses];   //邻接表
        for (int i = 0; i < numCourses; i++) {
            adj[i] = new HashSet<>();
        }

    	for (int[] path : prerequisites) {
    		int to = path[0];		
    		int from = path[1];		//先修课程
    		indegrees[to] ++ ;	//统计入度
    		adj[from].add(to);	//建立邻接表
    	}
        
    	Queue<Integer> q = new LinkedList<Integer>();
        //入度为0的点加入队列作为起点
    	for (int i = 0; i < numCourses; i++) {
			if(indegrees[i] == 0)
				q.offer(i);
		}
        
    	int count = 0;
    	while(!q.isEmpty()) {
    		int course = q.poll();	
    		count++ ;
    		for (Integer after: adj[course]) {
                //将入度为0的点删除后，其邻接点的入度均减1
				indegrees[after]-- ;
				if(indegrees[after] == 0)
					q.offer(after);
			}
    	}
        
        //出队列点的数目是否等于课程数
    	return count == numCourses;
    }
}

210. 课程表 II

题没变，输出拓扑排列的顺序，特判错了好多次。。。

还有就是，要求返回数组的，以后一律用list做，最后遍历一遍链表就行，用数组判断是否越界太繁琐了。

class Solution {	
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        if (numCourses <= 0) {
            return new int[] {0};
        }
    	int[] indegrees = new int[numCourses]; //入度数组
        HashSet<Integer>[] adj = new HashSet[numCourses];   //邻接表
        for (int i = 0; i < numCourses; i++) {
            adj[i] = new HashSet<>();
        }

    	for (int[] path : prerequisites) {
    		int to = path[0];		
    		int from = path[1];		//先修课程
    		indegrees[to] ++ ;	//统计入度
    		adj[from].add(to);	//建立邻接表
    	}
        
        List<Integer> res = new LinkedList<Integer>();
    	Queue<Integer> q = new LinkedList<Integer>();
        //入度为0的点加入队列作为起点
    	for (int i = 0; i < numCourses; i++) {
			if(indegrees[i] == 0) 
				q.offer(i);
		}

    	int count = 0;
    	while(!q.isEmpty()) {
    		int course = q.poll();
    		res.add(course);
    		count++ ;
    		for (Integer after: adj[course]) {
                //将入度为0的点删除后，其邻接点的入度均减1
				indegrees[after]-- ;
				if(indegrees[after] == 0)
					q.offer(after);
			}
    	}
        int[] answer = new int[res.size()];
        for (int i = 0; i < res.size(); i++) {
			answer[i] = res.get(i);
		}
        //出队列点的数目是否等于课程数
    	if(count == numCourses)
    		return answer;
    	else 
    		return new int[] {};
    }
}