122. Best Time to Buy and Sell Stock II

Method 1

You are given an integer array prices where prices[i] is the price of a given stock on the ith day.
On each day, you may decide to buy and/or sell the stock. You can only hold at most one share of the stock at any time. However, you can buy it then immediately sell it on the same day.
Find and return the maximum profit you can achieve.

Example 1

Input: prices = [ 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.
Total profit is 4 + 3 = 7.

Example 2

Input: prices = [ 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.
Total profit is 4.

Example 3

Input: prices = [ 7,6,4,3,1 ]
Output: 0
Explanation: There is no way to make a positive profit, so we never buy the stock to achieve the maximum profit of 0.

Method 1

【O(n) time | O(1) space】

package Leetcode.SlideWindow;

/**
 * @author zhengstars
 * @date 2024/03/21
 */
public class BestTimeToBuyAndSellStockII {
    public static int maxProfit(int[] prices) {
        // Edge case: If array is null or has less than 2 elements, no profit can be made
        if(prices == null || prices.length <= 1){
            return 0;
        }

        // Variable to keep track of total profit from all transactions
        int totalProfit = 0;

        // Iterate through the array starting from index 1 to compare with previous day
        for (int i = 1; i < prices.length; i++) {
            // If today's price is higher than yesterday's price
            // We can make a profit by buying yesterday and selling today
            if(prices[i] > prices[i -1]){
                // Add the price difference to our total profit
                // This is equivalent to buying at yesterday's price and selling at today's price
                totalProfit += prices[i] - prices[i -1];
            }
        }

        return totalProfit;
    }

    /**
     * Main method to test the solution with example cases
     */
    public static void main(String[] args) {

        int[] prices1 = { 7,1,5,3,6,4 };
        System.out.println(maxProfit(prices1));  // Output: 7

        int[] prices2 = { 7,6,4,3,1 };
        System.out.println(maxProfit(prices2));  // Output: 0
    }
}

Method 1

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