LeetCode-in-All

207. Course Schedule

Medium

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] = [ai, bi] 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 <= 105
• 0 <= prerequisites.length <= 5000
• prerequisites[i].length == 2
• 0 <= ai, bi < numCourses
• All the pairs prerequisites[i] are unique.

Solution

class Solution {
    fun canFinish(numCourses: Int, prerequisites: Array<IntArray>): Boolean {
        val adj: Array<ArrayList<Int>> = Array(numCourses) { ArrayList() }
        for (pre in prerequisites) {
            adj[pre[1]].add(pre[0])
        }
        val colors = IntArray(numCourses)
        for (i in 0 until numCourses) {
            if (colors[i] == WHITE && adj[i].isNotEmpty() && hasCycle(adj, i, colors)) {
                return false
            }
        }
        return true
    }

    private fun hasCycle(adj: Array<ArrayList<Int>>, node: Int, colors: IntArray): Boolean {
        colors[node] = GRAY
        for (nei in adj[node]) {
            if (colors[nei] == GRAY) {
                return true
            }
            if (colors[nei] == WHITE && hasCycle(adj, nei, colors)) {
                return true
            }
        }
        colors[node] = BLACK
        return false
    }

    companion object {
        private const val WHITE = 0
        private const val GRAY = 1
        private const val BLACK = 2
    }
}

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