leetcode-253-meeting-rooms-ii.swift

/**

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


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.
Space complexity O(n) where n is the amount of elements in intervals.
*/
func minMeetingRooms(_ intervals: [[Int]]) -> Int {
        
    // If we have no meetings, we need no rooms
    guard !intervals.isEmpty else {
        return 0
    } 
    
    // If we have 1 meeting, we need 1 room
    guard intervals.count > 1 else {
        return 1
    }
    
    // Sort by start time
    var sorted = intervals.sorted { interval1, interval2 in
        return interval1[0] < interval2[0]
    }     
    
    /**
    Get a min heap with sorting by end time
    and init the heap with the first element of sorted.

    Heap implementation taken from: 
    https://github.com/raywenderlich/swift-algorithm-club/blob/master/Heap/Heap.swift
    */
    var minHeap: Heap<[Int]> = Heap([sorted.first!], areSorted: { $0[1] < $1[1] } )

    // Loop through the remaining elements
    for i in 1..<sorted.count {

        // Get the element
        var current = sorted[i]

        // And the root of the heap
        var heap = minHeap.removeRoot()!
        
        /**
        Check if the current element
        starts before the last root of the heap ended
        */
        if current[0] >= heap[1] {

            /**
            ... if not, then adjust the end 
            time of the heap element
            */
            heap[1] = current[1]
        } else {

            // If it does, add it to the heap
            minHeap.insert(current)
        }
        
        // Add the removed element back
        minHeap.insert(heap)
    }
    
    // And return the heap count
    return minHeap.count
}