3
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https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/

Please review for performance.

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

Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times).

Note: You may not engage in multiple transactions at the same time (i.e., you must sell the stock before you buy again).

Example 1:

Input: [7,1,5,3,6,4]
Output: 7
Explanation: Buy on day 2 (price = 1) and sell on day 3 (price = 5), profit = 5-1 = 4.
             Then buy on day 4 (price = 3) and sell on day 5 (price = 6), profit = 6-3 = 3.
Example 2:

Input: [1,2,3,4,5]
Output: 4
Explanation: Buy on day 1 (price = 1) and sell on day 5 (price = 5), profit = 5-1 = 4.
             Note that you cannot buy on day 1, buy on day 2 and sell them later, as you are
             engaging multiple transactions at the same time. You must sell before buying again.
Example 3:

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

namespace ArrayQuestions
{
    /// <summary>
    /// https://leetcode.com/explore/featured/card/top-interview-questions-easy/92/array/564/
    /// </summary>
    [TestClass]
    public class BestTimetoBuyandSellStockII
    {
        [TestMethod]
        public void AnswerIs7()
        {
            int[] prices = {7, 1, 5, 3, 6, 4};
            Assert.AreEqual(7, MaxProfit(prices));
        }

        public int MaxProfit(int[] prices)
        {
            int max = 0;
            for (int i = 0; i < prices.Length-1; i++)
            {
                if (prices[i] < prices[i + 1])
                {
                    max += prices[i + 1] - prices[i];
                }
            }
            return max;

        }
    }
}
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Performance

Your solution is a single pass with time complexity \$O(n)\$. It's a small variation of the proposed solution.

Conventions

  • use static when no instance state/operations are used
  • use a clear, non-abbreviated method name CalculateMaximumProfit
  • always guard against bad user input in public methods
  • prefer the use of var when the type is obvious when reading the code
  • prefer whitespace between operands and operators prices.Length - 1
  • clean up waisted whitespace (in this case, after the return statement)

applied to your code:

 public static int CalculateMaximumProfit(int[] prices)
 {
     prices = prices ?? throw new ArgumentNullException(nameof(prices));
     var max = 0;
     for (var i = 0; i < prices.Length - 1; i++)
     {
         if (prices[i] < prices[i + 1])
         {
             max += prices[i + 1] - prices[i];
         }
     }
     return max;
 }
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