Swift Xcode implementation of InterviewCake 'HiCal' calendar event merging using immutable Meeting objects. It also doesn't start by making a copy of the entire Meetings array, saving one linear iteration of O(n). ⚠️ Spoiler alert: this is an actual answer to the https://www.interviewcake.com/question/swift/merging-ranges question, so no peeking!

InterviewCake-Meeting.swift

//
//  Meeting.swift
//

import Foundation

class Meeting: CustomStringConvertible {

    private(set) var startTime: Int
    private(set) var endTime: Int

    init(startTime: Int, endTime: Int) {

        // number of 30 min blocks past 9:00 am
        self.startTime = startTime

        assert(endTime >= startTime) // Ensure that Meeting records are coherent
        self.endTime = max(startTime, endTime)
    }

    var description: String {
        return "(\(startTime), \(endTime))"
    }
}

extension Meeting: Equatable {
    static func ==(lhs: Meeting, rhs: Meeting) -> Bool {
        return lhs.startTime == rhs.startTime && lhs.endTime == rhs.endTime
    }
}

InterviewCake-MergeMeetingRange.swift

//
//  MergeMeetingRange.swift
//
//  Copyright © 2018 Xavier Schott
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.
//

import Foundation

class MergeMeetingRange {
    static func mergeRanges(_ source: [Meeting]) -> [Meeting] {
        var result = [Meeting]()
        
        if !source.isEmpty {
            let byStartingTime = source.sorted() { (lhs, rhs) -> Bool in
                return lhs.startTime <= rhs.startTime
            }
            
            var lastStartTime = byStartingTime[0].startTime
            var lastEndTime = byStartingTime[0].endTime
            var waitingForGap = true
            
            byStartingTime.forEach { (meeting) in
                if meeting.startTime > lastEndTime {
                    // gap
                    result.append(Meeting(startTime: lastStartTime, endTime: lastEndTime))
                    lastStartTime = meeting.startTime
                    lastEndTime = meeting.endTime
                    waitingForGap = false
                } else {
                    // no gap
                    lastEndTime = max(lastEndTime, meeting.endTime)
                    waitingForGap = true
                }
            }
            if waitingForGap {
                result.append(Meeting(startTime: lastStartTime, endTime: lastEndTime))
            }
        }
        
        return result
    }
}

InterviewCake-MergeMeetingRangeTests.swift

//
//  MergeMeetingRangeTests.swift
//
//  Copyright © 2018 Xavier Schott
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.
//

import XCTest

class MergeMeetingRangeTests: XCTestCase {
    func testMergeMeetingRangeEmpty() {
        let test = [Meeting]()

        let expected = [Meeting]()

        let result = MergeMeetingRange.mergeRanges(test)
        XCTAssertEqual(expected, result)
    }

    func testMergeMeetingRangeSingleEntry() {
        let test = [
            Meeting(startTime: 7, endTime: 9)
        ]

        let expected =   [
            Meeting(startTime: 7, endTime: 9)
        ]

        let result = MergeMeetingRange.mergeRanges(test)
        XCTAssertEqual(expected, result)
    }

    func testMergeMeetingRangeFromQuestion() {
        let test = [
            Meeting(startTime: 0, endTime: 1), Meeting(startTime: 3, endTime: 5),
            Meeting(startTime: 4, endTime: 8), Meeting(startTime: 10, endTime: 12),
            Meeting(startTime: 9, endTime: 10)
        ]

        let expected =   [
            Meeting(startTime: 0, endTime: 1),
            Meeting(startTime: 3, endTime: 8),
            Meeting(startTime: 9, endTime: 12)
        ]

        let result = MergeMeetingRange.mergeRanges(test)
        XCTAssertEqual(expected, result)
    }

    func testMergeMeetingRangeFromAnswer() {
        let test = [
            Meeting(startTime: 1, endTime: 10),
            Meeting(startTime: 2, endTime: 6),
            Meeting(startTime: 3, endTime: 5),
            Meeting(startTime: 7, endTime: 9)
        ]

        let expected =   [
            Meeting(startTime: 1, endTime: 10)
        ]

        let result = MergeMeetingRange.mergeRanges(test)
        XCTAssertEqual(expected, result)
    }

    func testMergeMeetingRangeWithMixedValues() {
        let test = [
            Meeting(startTime: 0, endTime: 3), Meeting(startTime: 3, endTime: 5),
            Meeting(startTime: 4, endTime: 8), Meeting(startTime: 10, endTime: 12),
            Meeting(startTime: 9, endTime: 10), Meeting(startTime: 0, endTime: 1),
            Meeting(startTime: 1, endTime: 2),
            Meeting(startTime: 20, endTime: 21), Meeting(startTime: 21, endTime: 22)
        ]

        let expected =   [
            Meeting(startTime: 0, endTime: 8),
            Meeting(startTime: 9, endTime: 12),
            Meeting(startTime: 20, endTime: 22)
        ]

        let result = MergeMeetingRange.mergeRanges(test)
        XCTAssertEqual(expected, result)
    }
}

This implementation of mergeRanges allows for the Meeting class to be immutable. It's source was modified at

• private(set) var startTime: Int
• private(set) var endTime: Int

which wasn't a requirement.