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Maximum profit by buying and selling a share at most twice
  • Difficulty Level : Hard
  • Last Updated : 16 Feb, 2021

In daily share trading, a buyer buys shares in the morning and sells them on the same day. If the trader is allowed to make at most 2 transactions in a day, whereas the second transaction can only start after the first one is complete (Sell->buy->sell->buy). Given stock prices throughout the day, find out the maximum profit that a share trader could have made.

Examples: 

Input:   price[] = {10, 22, 5, 75, 65, 80}
Output:  87
Trader earns 87 as sum of 12, 75 
Buy at 10, sell at 22, 
Buy at 5 and sell at 80

Input:   price[] = {2, 30, 15, 10, 8, 25, 80}
Output:  100
Trader earns 100 as sum of 28 and 72
Buy at price 2, sell at 30, buy at 8 and sell at 80

Input:   price[] = {100, 30, 15, 10, 8, 25, 80};
Output:  72
Buy at price 8 and sell at 80.

Input:   price[] = {90, 80, 70, 60, 50}
Output:  0
Not possible to earn.

A Simple Solution is to consider every index ‘i’ and do the following 

Max profit with at most two transactions =
       MAX {max profit with one transaction and subarray price[0..i] +
            max profit with one transaction and subarray price[i+1..n-1]  }
i varies from 0 to n-1.

Maximum possible using one transaction can be calculated using the following O(n) algorithm 
The maximum difference between two elements such that the larger element appears after the smaller number
The time complexity of the above simple solution is O(n2).

We can do this O(n) using the following Efficient Solution. The idea is to store the maximum possible profit of every subarray and solve the problem in the following two phases.



1) Create a table profit[0..n-1] and initialize all values in it 0.
2) Traverse price[] from right to left and update profit[i] such that profit[i] stores maximum profit achievable from one transaction in subarray price[i..n-1]
3) Traverse price[] from left to right and update profit[i] such that profit[i] stores maximum profit such that profit[i] contains maximum achievable profit from two transactions in subarray price[0..i].
4) Return profit[n-1]

To do step 2, we need to keep track of the maximum price from right to left side, and to do step 3, we need to keep track of the minimum price from left to right. Why we traverse in reverse directions? The idea is to save space, in the third step, we use the same array for both purposes, maximum with 1 transaction and maximum with 2 transactions. After iteration i, the array profit[0..i] contains the maximum profit with 2 transactions and profit[i+1..n-1] contains profit with two transactions.

Below are the implementations of the above idea.

C++




// C++ program to find maximum
// possible profit with at most
// two transactions
#include <bits/stdc++.h>
using namespace std;
 
// Returns maximum profit with
// two transactions on a given
// list of stock prices, price[0..n-1]
int maxProfit(int price[], int n)
{
    // Create profit array and
    // initialize it as 0
    int* profit = new int[n];
    for (int i = 0; i < n; i++)
        profit[i] = 0;
 
    /* Get the maximum profit with
       only one transaction
       allowed. After this loop,
       profit[i] contains maximum
       profit from price[i..n-1]
       using at most one trans. */
    int max_price = price[n - 1];
    for (int i = n - 2; i >= 0; i--) {
        // max_price has maximum
        // of price[i..n-1]
        if (price[i] > max_price)
            max_price = price[i];
 
        // we can get profit[i] by taking maximum of:
        // a) previous maximum, i.e., profit[i+1]
        // b) profit by buying at price[i] and selling at
        //    max_price
        profit[i]
            = max(profit[i + 1], max_price - price[i]);
    }
 
    /* Get the maximum profit with two transactions allowed
       After this loop, profit[n-1] contains the result */
    int min_price = price[0];
    for (int i = 1; i < n; i++) {
        // min_price is minimum price in price[0..i]
        if (price[i] < min_price)
            min_price = price[i];
 
        // Maximum profit is maximum of:
        // a) previous maximum, i.e., profit[i-1]
        // b) (Buy, Sell) at (min_price, price[i]) and add
        //    profit of other trans. stored in profit[i]
        profit[i] = max(profit[i - 1],
                        profit[i] + (price[i] - min_price));
    }
    int result = profit[n - 1];
 
    delete[] profit; // To avoid memory leak
 
    return result;
}
 
// Driver code
int main()
{
    int price[] = { 2, 30, 15, 10, 8, 25, 80 };
    int n = sizeof(price) / sizeof(price[0]);
    cout << "Maximum Profit = " << maxProfit(price, n);
    return 0;
}

Java




class Profit {
    // Returns maximum profit
    // with two transactions on a
    // given list of stock prices,
    // price[0..n-1]
    static int maxProfit(int price[], int n)
    {
        // Create profit array
        // and initialize it as 0
        int profit[] = new int[n];
        for (int i = 0; i < n; i++)
            profit[i] = 0;
 
        /* Get the maximum profit
           with only one transaction
           allowed. After this loop,
           profit[i] contains
           maximum profit from
           price[i..n-1] using at most
           one trans. */
        int max_price = price[n - 1];
        for (int i = n - 2; i >= 0; i--) {
            // max_price has maximum
            // of price[i..n-1]
            if (price[i] > max_price)
                max_price = price[i];
 
            // we can get profit[i]
            // by taking maximum of:
            // a) previous maximum,
            // i.e., profit[i+1]
            // b) profit by buying
            // at price[i] and selling
            // at
            //    max_price
            profit[i] = Math.max(profit[i + 1],
                                 max_price - price[i]);
        }
 
        /* Get the maximum profit
           with two transactions allowed
           After this loop, profit[n-1]
           contains the result
         */
        int min_price = price[0];
        for (int i = 1; i < n; i++) {
            // min_price is minimum
            // price in price[0..i]
            if (price[i] < min_price)
                min_price = price[i];
 
            // Maximum profit is maximum of:
            // a) previous maximum, i.e., profit[i-1]
            // b) (Buy, Sell) at (min_price, price[i]) and
            // add
            // profit of other trans.
            // stored in profit[i]
            profit[i] = Math.max(
                profit[i - 1],
                profit[i] + (price[i] - min_price));
        }
        int result = profit[n - 1];
        return result;
    }
 
    // Driver Code
    public static void main(String args[])
    {
        int price[] = { 2, 30, 15, 10, 8, 25, 80 };
        int n = price.length;
        System.out.println("Maximum Profit = "
                           + maxProfit(price, n));
    }
 
} /* This code is contributed by Rajat Mishra */

Python




# Returns maximum profit with
# two transactions on a given
# list of stock prices price[0..n-1]
 
 
def maxProfit(price, n):
 
    # Create profit array and initialize it as 0
    profit = [0]*n
 
    # Get the maximum profit
    # with only one transaction
    # allowed. After this loop,
    # profit[i] contains maximum
    # profit from price[i..n-1]
    # using at most one trans.
    max_price = price[n-1]
 
    for i in range(n-2, 0, -1):
 
        if price[i] > max_price:
            max_price = price[i]
 
        # we can get profit[i] by
        # taking maximum of:
        # a) previous maximum,
        # i.e., profit[i+1]
        # b) profit by buying at
        # price[i] and selling at
        #    max_price
        profit[i] = max(profit[i+1], max_price - price[i])
 
    # Get the maximum profit
    # with two transactions allowed
    # After this loop, profit[n-1]
    # contains the result
    min_price = price[0]
 
    for i in range(1, n):
 
        if price[i] < min_price:
            min_price = price[i]
 
        # Maximum profit is maximum of:
        # a) previous maximum,
        # i.e., profit[i-1]
        # b) (Buy, Sell) at
        # (min_price, A[i]) and add
        #  profit of other trans.
        # stored in profit[i]
        profit[i] = max(profit[i-1], profit[i]+(price[i]-min_price))
 
    result = profit[n-1]
 
    return result
 
 
# Driver function
price = [2, 30, 15, 10, 8, 25, 80]
print "Maximum profit is", maxProfit(price, len(price))
 
# This code is contributed by __Devesh Agrawal__

C#




// C# program to find maximum possible profit
// with at most two transactions
using System;
 
class GFG {
 
    // Returns maximum profit with two
    // transactions on a given list of
    // stock prices, price[0..n-1]
    static int maxProfit(int[] price, int n)
    {
 
        // Create profit array and initialize
        // it as 0
        int[] profit = new int[n];
        for (int i = 0; i < n; i++)
            profit[i] = 0;
 
        /* Get the maximum profit with only
        one transaction allowed. After this
        loop, profit[i] contains maximum
        profit from price[i..n-1] using at
        most one trans. */
        int max_price = price[n - 1];
 
        for (int i = n - 2; i >= 0; i--) {
 
            // max_price has maximum of
            // price[i..n-1]
            if (price[i] > max_price)
                max_price = price[i];
 
            // we can get profit[i] by taking
            // maximum of:
            // a) previous maximum, i.e.,
            // profit[i+1]
            // b) profit by buying at price[i]
            // and selling at max_price
            profit[i] = Math.Max(profit[i + 1],
                                 max_price - price[i]);
        }
 
        /* Get the maximum profit with two
        transactions allowed After this loop,
        profit[n-1] contains the result */
        int min_price = price[0];
 
        for (int i = 1; i < n; i++) {
 
            // min_price is minimum price in
            // price[0..i]
            if (price[i] < min_price)
                min_price = price[i];
 
            // Maximum profit is maximum of:
            // a) previous maximum, i.e.,
            // profit[i-1]
            // b) (Buy, Sell) at (min_price,
            // price[i]) and add profit of
            // other trans. stored in
            // profit[i]
            profit[i] = Math.Max(
                profit[i - 1],
                profit[i] + (price[i] - min_price));
        }
        int result = profit[n - 1];
 
        return result;
    }
 
    // Driver code
    public static void Main()
    {
        int[] price = { 2, 30, 15, 10, 8, 25, 80 };
        int n = price.Length;
 
        Console.Write("Maximum Profit = "
                      + maxProfit(price, n));
    }
}
 
// This code is contributed by nitin mittal.

PHP




<?php
// PHP program to find maximum
// possible profit with at most
// two transactions
 
// Returns maximum profit with
// two transactions on a given
// list of stock prices, price[0..n-1]
function maxProfit($price, $n)
{
    // Create profit array and
    // initialize it as 0
    $profit = array();
    for ($i = 0; $i < $n; $i++)
        $profit[$i] = 0;
 
    // Get the maximum profit with
    // only one transaction allowed.
    // After this loop, profit[i]
    // contains maximum profit from
    // price[i..n-1] using at most
    // one trans.
    $max_price = $price[$n - 1];
    for ($i = $n - 2; $i >= 0; $i--)
    {
        // max_price has maximum
        // of price[i..n-1]
        if ($price[$i] > $max_price)
            $max_price = $price[$i];
 
        // we can get profit[i] by
        // taking maximum of:
        // a) previous maximum,
        //    i.e., profit[i+1]
        // b) profit by buying at
        // price[i] and selling at
        // max_price
        if($profit[$i + 1] >
           $max_price-$price[$i])
        $profit[$i] = $profit[$i + 1];
        else
        $profit[$i] = $max_price -
                      $price[$i];
    }
 
    // Get the maximum profit with
    // two transactions allowed.
    // After this loop, profit[n-1]
    // contains the result
    $min_price = $price[0];
    for ($i = 1; $i < $n; $i++)
    {
        // min_price is minimum
        // price in price[0..i]
        if ($price[$i] < $min_price)
            $min_price = $price[$i];
 
        // Maximum profit is maximum of:
        // a) previous maximum,
        //    i.e., profit[i-1]
        // b) (Buy, Sell) at (min_price,
        //     price[i]) and add
        // profit of other trans.
        // stored in profit[i]
        $profit[$i] = max($profit[$i - 1],
                          $profit[$i] +
                         ($price[$i] - $min_price));
    }
    $result = $profit[$n - 1];
    return $result;
}
 
// Driver Code
$price = array(2, 30, 15, 10,
               8, 25, 80);
$n = sizeof($price);
echo "Maximum Profit = ".
      maxProfit($price, $n);
     
// This code is contributed
// by Arnab Kundu
?>
Output
Maximum Profit = 100

The time complexity of the above solution is O(n). 
Algorithmic Paradigm: Dynamic Programming 

There is one more approach for calculating this problem using the Valley-Peak approach i.e. take a variable profit and initialize it with zero and then traverse through the array of price[] from (i+1)th position whenever the initial position value is greater than the previous value add it to variable profit.

C++




#include <iostream>
using namespace std;
 
int main()
{
    int price[] = { 2, 30, 15, 10, 8, 25, 80 };
    int n = 7;
   
    // adding array
    int profit = 0;
   
    // Initializing variable
    // valley-peak approach
    /*
                       80
                       /
        30            /
       /  \          25
      /    15       /
     /      \      /
    2        10   /
               \ /
                8
     */
    for (int i = 1; i < n; i++)
    {
       
        // traversing through array from (i+1)th
        // position
        int sub = price[i] - price[i - 1];
        if (sub > 0)
            profit += sub;
    }
 
    cout << "Maximum Profit=" << profit;
    return 0;
}
 
// This code is contributed by RohitOberoi.

Java




import java.io.*;
import java.util.*;
 
class GFG {
    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
 
        int price[] = { 2, 30, 15, 10, 8, 25, 80 };
        // adding array
        int profit = 0;
        // Initializing variable
        // valley-peak approach
        /*
                           80
                           /
            30            /
           /  \          25
          /    15       /
         /      \      /
        2        10   /
                   \ /
                    8
         */
        for (int i = 1; i < price.length; i++) {
            // traversing through array from (i+1)th
            // position
            int sub = price[i] - price[i - 1];
            if (sub > 0)
                profit += sub;
        }
        System.out.print("Maximum Profit=" + profit);
    }
}

C#




using System;
using System.Collections.Generic;
class GFG {
 
    static void Main()
    {
 
        int[] price = { 2, 30, 15, 10, 8, 25, 80 };
 
        // adding array
        int profit = 0;
 
        // Initializing variable
        // valley-peak approach
        /*
                           80
                           /
            30            /
           /  \          25
          /    15       /
         /      \      /
        2        10   /
                   \ /
                    8
         */
        for (int i = 1; i < price.Length; i++) {
 
            // traversing through array from (i+1)th
            // position
            int sub = price[i] - price[i - 1];
            if (sub > 0)
                profit += sub;
        }
        Console.WriteLine("Maximum Profit=" + profit);
    }
}
 
// This code is contributed by divyeshrabadiya07
Output
Maximum Profit=100

The time and space complexity is O(n) and O(1) respectively.

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