09. Maximum slice problem. MaxProfit.swift

import Foundation
import Glibc

// Solution @ Sergey Leschev, Belarusian State University

// 09. Maximum slice problem. MaxProfit.

// An array A consisting of N integers is given. It contains daily prices of a stock share for a period of N consecutive days. If a single share was bought on day P and sold on day Q, where 0 ≤ P ≤ Q < N, then the profit of such transaction is equal to A[Q] − A[P], provided that A[Q] ≥ A[P]. Otherwise, the transaction brings loss of A[P] − A[Q].

// For example, consider the following array A consisting of six elements such that:
//   A[0] = 23171
//   A[1] = 21011
//   A[2] = 21123
//   A[3] = 21366
//   A[4] = 21013
//   A[5] = 21367
// If a share was bought on day 0 and sold on day 2, a loss of 2048 would occur because A[2] − A[0] = 21123 − 23171 = −2048. If a share was bought on day 4 and sold on day 5, a profit of 354 would occur because A[5] − A[4] = 21367 − 21013 = 354. Maximum possible profit was 356. It would occur if a share was bought on day 1 and sold on day 5.

// Write a function,
// class Solution { public int solution(int[] A); }
// that, given an array A consisting of N integers containing daily prices of a stock share for a period of N consecutive days, returns the maximum possible profit from one transaction during this period. The function should return 0 if it was impossible to gain any profit.

// For example, given array A consisting of six elements such that:
//   A[0] = 23171
//   A[1] = 21011
//   A[2] = 21123
//   A[3] = 21366
//   A[4] = 21013
//   A[5] = 21367
// the function should return 356, as explained above.

// Write an efficient algorithm for the following assumptions:
// N is an integer within the range [0..400,000];
// each element of array A is an integer within the range [0..200,000].

public func solution(_ A: inout [Int]) -> Int {
    var profit = 0
    var sharePrice = Int.max
    
    for price in A {
        if price < sharePrice {
            sharePrice = price
        } else {
            profit = max(profit, price - sharePrice)
        }
    }
    return profit
}