leetcode-252-meeting-room.swift

/**

Copyright (c) 2020 David Seek, LLC

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OR OTHER DEALINGS IN THE SOFTWARE.





https://leetcode.com/problems/meeting-rooms/


Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), determine if a person could attend all meetings.

Example 1:

Input: [[0,30],[5,10],[15,20]]
Output: false
Example 2:

Input: [[7,10],[2,4]]
Output: true
NOTE: input types have been changed on April 15, 2019. Please reset to default code definition to get new method signature.

*/

/**
Big O Annotation
Time complexity O(n log n) where n is the amount of elements in intervals.
The sorting costs us O(n log n)

I tried implemetnation with PriorityQueue (Heap), same runtime on leetcode.
Although technically the complexity is O(log n) with a Heap.
But since it's like 100 lines more code as Swift does not 
come with native Heaps, we might as well just go with the Stack.

Space complexity O(n) where n is the amount of elements in intervals.
*/
func canAttendMeetings(_ intervals: [[Int]]) -> Bool {
        
    // If we have less than 2 meetings, then yes, all good
    guard intervals.count > 1 else {
        return true
    }
    
    // Sort by meeting end time descending
    let sorted = intervals.sorted { interval1, interval2 in
        return interval1[1] > interval2[1]
    }
    
    // Get a stack
    var stack = Stack(sorted)

    // And pop the first meeting
    var last: [Int] = stack.pop()!
    
    // While the stack is not empty...
    while !stack.isEmpty {
        
        // Pop the last element
        let popped = stack.pop()!
        
        /**
        And compare if the new meeting starts 
        before the old meeting did end
        */
        if last[1] > popped[0] {
            return false
        }
        
        // Update last
        last = popped
    }
    
    /**
    If we fell all the way through,
    then we're good to go with that schedule.
    */
    return true
}

struct Stack {
    private var stack: [[Int]]
    public var isEmpty: Bool { return stack.isEmpty }
    init(_ array: [[Int]]) { stack = array }
    public mutating func pop() -> [Int]? { return stack.popLast() }
}