Find-Maximum-Profit-FAST.swift

// Run in an Xcode Playground

let prices: [Double] = [6,0,-1,10]
let prices2: [Double] = []
let prices3: [Double] = [1]
let prices4: [Double] = [6,0,-1,10,11,12,1,5,200,-2,200,32,-322,0,124,432,-1,-1000,1]

func solution(prices: [Double]) -> Double {
    
    // the idea is to find the max between two key differences in the array:
    // 1 - the largest number in the array minus the smallest number that precedes it
    // 2 - the smallest number in the array minus the largest number that follows it (then times by -1 to fix the sign)
    guard let largestNumber = prices.max(),
        let indexOfLastOccurrenceOfLargestNumber = prices.lastIndex(of: largestNumber),
        let smallestNumberPreviousToLastOccurenceOfLargestNumber = prices.prefix(through: indexOfLastOccurrenceOfLargestNumber).min(),
        let smallestNumber = prices.min(),
        let indexOfFirstOccurrenceOfSmallestNumber = prices.firstIndex(of: smallestNumber),
        let largestNumberAfterFirstOccurenceOfSmallestNumber = prices.suffix(from: indexOfFirstOccurrenceOfSmallestNumber).max()
        else {
        return 0
    }
    
    let largestNumberSubtractingSmallestPreviousNumber = largestNumber - smallestNumberPreviousToLastOccurenceOfLargestNumber
    let smallestNumberSubtractingLargestFollowingNumber = smallestNumber - largestNumberAfterFirstOccurenceOfSmallestNumber // don't forget to flip the sign later since this will be negative due to the numbers' ordering in the naming convention
    
    return max(largestNumberSubtractingSmallestPreviousNumber, -smallestNumberSubtractingLargestFollowingNumber)
}

solution(prices: prices) == 11
solution(prices: prices2) == 0
solution(prices: prices3) == 0
solution(prices: prices4) == 1001