[Swift]LeetCode121. 买卖股票的最佳时机 I | Best Time to Buy and Sell Stock

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Say you have an array for which the ith element is the price of a given stock on day i.

If you were only permitted to complete at most one transaction (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Note that you cannot sell a stock before you buy one.

Example 1:

Input: [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.
             Not 7-1 = 6, as selling price needs to be larger than buying price.

Example 2:

Input: [7,6,4,3,1]
Output: 0
Explanation: In this case, no transaction is done, i.e. max profit = 0.

---

给定一个数组，它的第 i 个元素是一支给定股票第 i 天的价格。

若是你最多只容许完成一笔交易（即买入和卖出一支股票），设计一个算法来计算你所能获取的最大利润。

注意你不能在买入股票前卖出股票。

示例 1:

输入: [7,1,5,3,6,4]
输出: 5
解释: 在第 2 天（股票价格 = 1）的时候买入，在第 5 天（股票价格 = 6）的时候卖出，最大利润 = 6-1 = 5 。
     注意利润不能是 7-1 = 6, 由于卖出价格须要大于买入价格。

示例 2:

输入: [7,6,4,3,1]
输出: 0
解释: 在这种状况下, 没有交易完成, 因此最大利润为 0。

---

 1 class Solution {
 2     func maxProfit(_ prices: [Int]) -> Int {
 3         if prices == nil || prices.count == 0
 4         {
 5             return 0
 6         }
 7         let counts = prices.count
 8         var arr:Array = Array(repeating: 0, count: counts)  
 9         var minPrice = prices[0]
10         for i in 1..<counts
11         {
12             minPrice = (minPrice < prices[i]) ? minPrice : prices[i]
13             arr[i] = (arr[i - 1] > (prices[i] - minPrice)) ? arr[i - 1] : (prices[i] - minPrice)
14         }
15         return arr[counts-1]
16     
17     }
18 }

---

20ms

 1 class Solution {
 2     func maxProfit(_ prices: [Int]) -> Int {
 3         guard prices.count > 0 else {
 4             return 0
 5         }
 6         
 7         var maxProfit = 0 
 8         var middleProfit = 0
 9         for i in 1..<prices.count {
10             middleProfit = max(prices[i] - prices[i - 1] + middleProfit, 0) 
11             maxProfit = max(maxProfit, middleProfit)
12         }
13         
14         return maxProfit
15     }
16 }

---

16ms

 1 class Solution {
 2     func maxProfit(_ prices: [Int]) -> Int {
 3         guard prices.count >= 2 else {
 4             return 0
 5         }
 6         var dif = 0
 7         var profit = 0
 8         for i in 1..<prices.count {
 9             dif = max(dif+prices[i]-prices[i-1],0)
10             profit = max(dif,profit)
11         }
12         return profit
13     }
14 }