Swift Xcode implementation of InterviewCake 'Temperature Tracker' with just ahead of time, making all methods perform in O(1). Mean is calculated when requested, still performing in O(1). ⚠️ Spoiler alert: this is an actual answer to the https://www.interviewcake.com/question/swift/temperature-tracker question, so no peeking!

InterviewCake-TemperatureTracker.swift

//
//  TemperatureTracker.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 TemperatureTracker
{
    var low: Int?
    var high: Int?
    var mean: Double? {
        return ((recordsCount > 0)
            ? Double(cumulativeTemperatures) / Double(recordsCount)
            : nil)
    }
    var mode: Int?

    private var occurences = [Int](repeatElement(0, count: 111))
    private var cumulativeTemperatures = 0
    private var recordsCount = 0
    private var largestOccurencesCount = 0

    func insert(value: Int) {
        assert( value >= 0 && value <= 110)
        if( value >= 0 && value <= 110) {
            // Min & Max
            low = min(low ?? Int.max, value)
            high = max(high ?? Int.min, value)
            
            // Mean
            cumulativeTemperatures += value
            recordsCount += 1
            
            // Mode
            occurences[value] += 1
            if occurences[value] > largestOccurencesCount {
                largestOccurencesCount = occurences[value]
                mode = value
            }
        }
    }
    
    func getMin() -> Int? {
        return low
    }
    
    func getMax() -> Int? {
        return high
    }
    
    func getMean() -> Double? {
        return mean
    }
    
    func getMode() -> Int? {
        return mode
    }
}

InterviewCake-TemperatureTrackerTests.swift

//
//  TemperatureTrackerTests.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 TemperatureTrackerTests: XCTestCase {
    func testTemperatureTracker() {
        let temps = [2,4,7,5,9,6,3,6]
        let tracker = TemperatureTracker()

        temps.forEach { (temp) in
            tracker.insert(value: temp)
        }

        let mean = (2.0 + 4.0 + 7.0 + 5.0 + 9.0 + 6.0 + 3.0 + 6.0) / 8.0
        XCTAssertEqual(9, tracker.getMax())
        XCTAssertEqual(2, tracker.getMin())
        XCTAssertEqual(6, tracker.getMode())
        XCTAssertEqual(mean, tracker.getMean())
    }
    
    func testPerformanceExample() {
        self.measure {
            let tracker = TemperatureTracker()
            for _ in 0 ... 16384 {
                tracker.insert(value: Int(arc4random_uniform(111)))
            }
        }
    }
    
}

Every function performs in O(1).

mean is calculated just-in-time, but the necessary nominator and denominator are accumulated at very little cost ahead-of-time. Running 2^14 iterations in the Unit Test only shows a marginal improvement, well within the margin of errors.

As noted, the properties get aren't necessary, and only provided because they are part of the solution's API.