[Swift] Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a given stock on the i$^{th}$ day.

You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Example 1:

Input: prices = [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.
Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Example 2:

Input: prices = [7,6,4,3,1]
Output: 0
Explanation: In this case, no transactions are done and the max profit = 0.

풀러가기

문제 이해

코드

func maxProfit(_ prices: [Int]) -> Int {
  var currentMax = 0 
  var posMax = 0
  for i in 1 ..< prices.count {
      currentMax += prices[i] - prices[i - 1]
      currentMax = max(0, currentMax)
      posMax = max(currentMax, posMax)
  }
  return posMax
}

풀이

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다른 분의 멋진 코드

func maxProfit(_ prices: [Int]) -> Int {
  var buyPrice = prices.first!
  var right = 0
  var profit = 0
  while right < prices.count {
      let price = prices[right]
      profit = max(profit, price - buyPrice)
      
      if price < buyPrice {
          buyPrice = price
      }
      
      right += 1
  }

  return profit
}

잘 배웠습니다.

• Kadane’s Algorithm to Maximum Sum Subarray Problem