Max Average Stock Price Solution Amazon OA 2022

Max Average Stock Price Solution

The interns at Amazon were asked to review the company’s stock value over a period. Given the stock prices of n months, the net price change for the i^th month is defined as the absolute difference between the average of stock prices for the first i months and for the remaining (n – i) months where 1≤ i <n. Note that these averages are rounded down to an integer.

Given an array of stock prices, find the month at which the net price change is minimum. If there are several such months, return the earliest month.

Note: The average of a set of integers here is defined as the sum of integers divided by the number of integers, rounded down to the nearest integer. For example, the average of [1, 2, 3, 4] is the floor of (1 + 2 + 3 + 4) / 4 = 2.5 and the floor of 2.5 is 2.

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Example

• stockPrice = [1, 3, 2, 3]
• The minimum net price change is 0, and it occurs in the 2^nd month. Return 2.

Function Description

• Complete the function findEarliestMonth the editor below.
• findEarliestMonth has the following parameter: int stockPrice[n]: the stock prices

Returns

int: the earliest month in which the net price change is minimum

Constraints

• 2 ≤ n ≤ 10⁵
• 1 ≤ stockPrice[i] ≤ 10⁹

Sample Input

STDIN               FUNCTION
------             ----------
5            ->    stockPrice[]
size n = 5
1            ->    stockPrice = [1, 3, 2, 4, 5]
3
2
4
5

Sample Output

2

Explanation

The net price change can be calculated as:

• Month 1: [1] and [3, 2, 4, 5], their respective averages, rounded down = 1 and 3, net price change = 2
• Month 2: [1, 3] and [2, 4, 5], averages = 2 and 3, net price change = 1
• Month 3: [1, 3, 2] and [4, 5], averages = 2 and 4, net price change = 2
• Month 4: [1, 3, 2, 4] and [5], averages = 2 and 5, net price change = 3

The minimum net price change is 1, and it occurs at month 2.

Sample Input 1

STDIN               FUNCTION
------             ----------
6            ->    stockPrice[]
size n = 6
1            ->    stockPrice = [1, 1, 1, 1, 1, 1]
1
1
1
1
1

Sample Output 1

1

Explanation

Since all the stock prices are all the same, the average is always 1 and the net price change is always 0. The earliest month this occurs is 1.

SOLUTION

Code: Max Average Stock Price Solution

public int maxAverage(int[] prices) {
 int n = prices.length;
 int[] postfix = new int[n];
 int sum = 0;
 for(int i=n-1; i>=0; i--) {
  postfix[i] = sum;
  sum+=prices[i];
 }
 int ans = 0;
 sum = 0;
 for(int i=0; i<n; i++) {
  sum+=prices[i];
  int preAverage = sum/(i-0+1);
  int postAverage = postfix[i]==0? postfix[i] : postfix[i]/(n-1-i);
  ans = Math.max(ans, Math.abs(preAverage-postAverage));
 }
 return ans;
}

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