LeetCode: Course Schedule II

Posted on August 5, 2019July 26, 2020 by braindenny

Course Schedule II

Similar Problems:

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, return the ordering of courses you should take to finish all courses.

There may be multiple correct orders, you just need to return one of them. If it is impossible to finish all courses, return an empty array.

Example 1:

Input: 2, [[1,0]] 
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished   
             course 0. So the correct course order is [0,1] .

Example 2:

Input: 4, [[1,0],[2,0],[3,1],[3,2]]
Output: [0,1,2,3] or [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both     
             courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. 
             So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3] .
Note:

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

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

Solution: topological sort + bfs

// Basic Ideas: topological sort + bfs
// Complexity: Time O(n+e), Space O(n+e)
func findOrder(numCourses int, prerequisites [][]int) []int {
    indegrees := make([]int, numCourses)
    edges := map[int]map[int]bool{}
    for _, p := range prerequisites {
        n1, n2 := p[0], p[1]
        if _, ok := edges[n2]; !ok {
            edges[n2] = map[int]bool{}
        }
        edges[n2][n1] = true
        indegrees[n1]++
    }
    res := []int{}
    queue := []int{}
    for i, v := range indegrees {
        if v == 0 {
            queue = append(queue, i)
            res = append(res, i)
        }
    }
    for len(queue) > 0 {
        l := []int{}
        for _, node1 := range queue {
            for node2, _ := range edges[node1] {
                indegrees[node2]--
                if indegrees[node2] == 0 {
                    l = append(l, node2)
                    res = append(res, node2)
                }
            }
        }
        queue = l
    }
    if len(res) != numCourses {
        res = []int{}
    }
    return res
}

Solution: topological sort + dfs

// https://code.dennyzhang.com/course-schedule-ii
// Basic Ideas: topological sort + dfs
// Complexity: Time O(n+e), Space O(n+e)
func dfs(node int, indegrees []int, edges map[int]map[int]bool, res *[]int) {
    if indegrees[node] != 0 {
        return
    }
    *res = append(*res, node)
    for node2, _ := range edges[node] {
        indegrees[node2]--
        dfs(node2, indegrees, edges, res)
    }
    // mark node as resolved
    indegrees[node] = -1
}

func findOrder(numCourses int, prerequisites [][]int) []int {
    indegrees := make([]int, numCourses)
    edges := map[int]map[int]bool{}
    for _, p := range prerequisites {
        n1, n2 := p[0], p[1]
        if _, ok := edges[n2]; !ok {
            edges[n2] = map[int]bool{}
        }
        edges[n2][n1] = true
        indegrees[n1]++
    }
    res := []int{}
    for node, _ := range indegrees {
        dfs(node, indegrees, edges, &res)
    }
    if len(res) != numCourses {
        res = []int{}
    }
    return res
}

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