LeetCode-in-Swift

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 {
    private let WHITE = 0
    private let GRAY = 1
    private let BLACK = 2

    func canFinish(_ numCourses: Int, _ prerequisites: [[Int]]) -> Bool {
        var adj = Array(repeating: [Int](), count: numCourses)
        for pre in prerequisites {
            adj[pre[1]].append(pre[0])
        }

        var colors = Array(repeating: WHITE, count: numCourses)
        for i in 0..<numCourses {
            if colors[i] == WHITE && !adj[i].isEmpty && hasCycle(adj, i, &colors) {
                return false
            }
        }
        return true
    }

    private func hasCycle(_ adj: [[Int]], _ node: Int, _ colors: inout [Int]) -> Bool {
        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
    }
}

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