Algorithms & Data structures, Problem #001
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Last Updated on September 6, 2022 by Deya Eldeen

After reading the very popular book, grokking algorithms,
Will be blogging about algorithms and data structures… the book is very informative and easy to digest.

Grokking Algorithms book

It’s advised you get yourself familiar with data structures before starting to solve problems…

Problem statement: return a pair of 2 distinct values (if any) that sum up to a target number, from a non empty array that has distinct integers.

Different Solutions with different time complexities

// Time: O(n^2)
func solution1(_ array: [Int], _ targetSum: Int) -> [Int] {
    for i in 0 ..< array.count-1 {
        for j in i+1 ..< array.count {
            if array[i] + array[j] == targetSum {
                return [array[i],array[j]]
            }
        }
    }
    return []
}

// Time: O(n^2)
func solution2(_ array: [Int], _ targetSum: Int) -> [Int] {
    for i in array {
        for j in array {
            if (i != j) && targetSum == (i + j) {
                return [i,j]
            }
        }
    }
    return []
}

// Time: O(n*log(n))
func solution3(_ array: [Int], _ targetSum: Int) -> [Int] {
    let sorted = array.sorted()
    var leftPointer = 0
    var rightPointer = sorted.count - 1
    while leftPointer < rightPointer {
        let leftMost = sorted[leftPointer]
        let rightMost = sorted[rightPointer]
        let currentSum = leftMost + rightMost
        if currentSum == targetSum {
            return [leftMost, rightMost]
        } else if currentSum < targetSum {
            leftPointer = leftPointer + 1
        } else if currentSum > targetSum {
            rightPointer = rightPointer - 1
        }
    }
    return []
}

// Time: O(n)
func solution4(_ array: [Int], _ targetSum: Int) -> [Int] {
    var numberDictionary = [Int: Bool]()
    for number in array {
        let mayMatch = targetSum - number
        if let exists = numberDictionary[mayMatch], exists {
            return [mayMatch, number]
        } else {
            numberDictionary[number] = true
        }
    }
    return []
}

I’m not going to explain each code, will leave analysis to you, doing a simple benchmark on 100,000 values array, we can see these results

solution1: 31.88 s.
solution2: 18.41 s.
solution3: 0.38 s.
solution4: 0.20 s. 🥇

Graphs depicting the complexity

functions used for benchmarking

func printTimeElapsedWhenRunningCode(title:String, operation:()->()) {
    let startTime = CFAbsoluteTimeGetCurrent()
    operation()
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print("Time elapsed for \(title): \(timeElapsed) s.")
}

func timeElapsedInSecondsWhenRunningCode(operation: ()->()) -> Double {
    let startTime = CFAbsoluteTimeGetCurrent()
    operation()
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    return Double(timeElapsed)
}

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