207. Course Schedule (Medium)

https://leetcode.com/problems/course-schedule/

There are a total of n courses you have to take, labeled from 0 to n-1.

Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]

Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?

Example 1:

Input: 2, [[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: 2, [[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.

Note:

  1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
  2. You may assume that there are no duplicate edges in the input prerequisites.

Hints

Solutions

class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        int[][] matrix = new int[numCourses][numCourses]; // i -> j
        int[] indegree = new int[numCourses];

        for (int i = 0; i < prerequisites.length; i++) {
            int ready = prerequisites[i][0];
            int pre = prerequisites[i][1];
            if (matrix[pre][ready] == 0) {
                indegree[ready]++;
            }
            matrix[pre][ready] = 1;
        }

        int count = 0;
        LinkedList<Integer> q = new LinkedList<>();
        for (int i = 0; i < indegree.length; i++) {
            if (indegree[i] == 0) { q.offer(i); }
        }
        while (!q.isEmpty()) {
            int course = q.poll();
            count++;
            for (int i = 0; i < numCourses; i++) {
                if (matrix[course][i] != 0) {
                    if (--indegree[i] == 0) {
                        q.offer(i);
                    }
                }
            }
        }
        return count == numCourses;
    }
}
Copyright © iovi.com 2017 all right reserved,powered by GitbookLast Modification: 2019-04-15 18:30:01

results matching ""

    No results matching ""