Stock Buy Sell to Maximize Profit

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Given an array of integers representing stock price on a single day, find max profit that can be earned by 1 transaction.
So you need to find a pair (buyDay,sellDay) where buyDay < = sellDay and it should maximize the profit. For example:

int arr[]={14, 12, 70, 15, 99, 65, 21, 90};
Max profit can be gain by buying on 1st day(0 based indexing) and sell on the 4th day.
Max profit = 99-12 =87

Stock Buy Sell to Maximize Profit Algorithm

Lets say we have array arr[] of stock prices.
We will track two variables :lowestPriceTillThatDayand maxProfit.

• lowestPriceTillThatDay will be initialise to arr[0].
• Iterate over stock price array arr[]
• If current element is greater than lowestPriceTillThatDay
  • calculate profit.
  • If profit is greater than maxProfit then update the maxProfit.
• If current element is lesser than lowestPriceTillThatDay
  • update lowestPriceTillThatDay with current element.
• We will get maxProfit in the end.

public static int calculateMaxProfit(int[] arr) {
        int lowestPriceTillThatDay = arr[0];
        int maxProfit = Integer.MIN_VALUE;
        for (int i = 0; i < arr.length; i++) {
            int profit = 0;
            if (arr[i] > lowestPriceTillThatDay) {
                profit = arr[i] - lowestPriceTillThatDay;
                if (profit > maxProfit) {
                    maxProfit = profit;
                }
            } else {
                lowestPriceTillThatDay = arr[i];
            }
        }
        return maxProfit;
    }

Java Program for Stock Buy Sell to Maximize Profit

package org.arpit.java2blog;

public class StockBuySellMain {

    public static void main(String[] args) {
        int arr[] = { 14, 12, 70, 15, 99, 65, 21, 90 };
        System.out.println("Maximum profit :" + calculateMaxProfit(arr));

    }

    public static int calculateMaxProfit(int[] arr) {
        int lowestPriceTillThatDay = arr[0];
        int maxProfit = Integer.MIN_VALUE;
        for (int i = 0; i < arr.length; i++) {
            int profit = 0;
            if (arr[i] > lowestPriceTillThatDay) {
                profit = arr[i] - lowestPriceTillThatDay;
                if (profit > maxProfit) {
                    maxProfit = profit;
                }
            } else {
                lowestPriceTillThatDay = arr[i];
            }
        }
        return maxProfit;
    }

}

When you run above program, you will get below output:

Maximum profit :87