reductionCost.swift

// # Challenge2

struct TestCase {
    let nums: [Int]
    let expectation: Int
}

let test1 = [3, 1, 2, 3] // => output: 9 | ok
/**
 3, 3 => 3
 6 => 6
 ===> 3 + 6 = 9
 */
let test2 = [4, 1, 2, 3, 4] // => output: 19 | something wrong
/**
 3, 4, 3 => 3
 3, 7    => 7
 10      => 10
 ===> 3 + 7 + 10 => 20
 */

let test3 = [5, 1, 3, 5, 6, 7] // => output: 48
/**
 5, 6, 7, 4 => 4
 7, 4, 11   => 11
 11, 11     => 11
 22         => 22
 ===> 4 + 11 + 11 + 22 => 48
 */
let cases = [
    TestCase(nums: test1, expectation: 9),
    TestCase(nums: test2, expectation: 19), // If you check it manually, you can't come out with output 19 -> It has to be 20, explanation above!
    TestCase(nums: test3, expectation: 48),
]

// My sent solution:
func reductionCost(num: [Int]) -> Int {
    var numbers = Array(num.dropFirst())
    var costs = [Int]()

    while numbers.count != 1 {
        let first = numbers.removeFirst()
        let second = numbers.removeFirst()
        let cost = first + second

        costs.append(cost)
        numbers.append(cost)
    }

    return costs.reduce(0, +)
}

// After timeout I've reworked a little bit:
func reductionCostOptimized(num: [Int]) -> Int {
    var nums = Array(num.dropFirst())
    var s = 0

    while nums.count > 1 {
        let first = nums.removeFirst()
        let second = nums.removeFirst()
        s += first + second
        nums.append(first + second)
    }
    return s
}

for test in cases {
    print("\n===")
    print("original: \(test.nums)\n- expectation: \(test.expectation)")

    print("Fn1: \(reductionCost(num: test.nums))")
    print("Fn2: \(reductionCostOptimized(num: test.nums))")
}

• Challenge#1 solution
• Challenge#3 solution