207. Course Schedule

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LeetCode

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Problem

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [a_i, b_i] indicates that you must take course bi first if you want to take course ai.

• For example, the pair [0, 1], indicates that to take course 0 you have to first take course 1.

Return true if you can finish all courses. Otherwise, return false.

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take. 
To take course 1 you should have finished course 0. So it is possible.

Example 2:

Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
Output: false
Explanation: There are a total of 2 courses to take. 
To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.

Constraints:

• 1 <= numCourses <= 2000
• 0 <= prerequisites.length <= 5000
• prerequisites[i].length == 2
• 0 <= ai, bi < numCourses
• All the pairs prerequisites[i] are unique.

Code

class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        int[] indegree = new int[numCourses];
        int res = numCourses;

        // 找入度为0，表示可以开始学习的课
        for(int[] pair : prerequisites){
            // [1,0] => 0->1
            indegree[pair[0]]++;
        }

        Queue<Integer> queue = new LinkedList<>();

        // 入度为0的加入，然后bfs
        for(int i = 0; i < indegree.length; i++){
            if(indegree[i] == 0){
                // 当前这个课已经可以学了
                queue.offer(i);
            }
        }

         while(!queue.isEmpty()){
            int curr = queue.poll();

            for(int[] pair : prerequisites){
                // 如果这门课已经可以学习了，就不加入queue
                if(indegree[pair[0]] == 0){
                    continue;
                }

                if(pair[1] == curr){
                    indegree[pair[0]]--;
                }
                // 这门课原来不能学习，现在可以学习了
                if(indegree[pair[0]] == 0){
                    queue.offer(pair[0]);
                }
            }
        }

        for(int i = 0; i < numCourses; i++){
            if(indegree[i] != 0){
                return false;
            }
        }

        return true;
    }
}