#2806

Easy

auto_awesomeMath-driven solution strategy

LeetCode Problem Workspace

Account Balance After Rounded Purchase

Given a purchase amount, calculate the account balance after rounding the purchase to the nearest multiple of 10 and deducting it.

math

Problem Statement

You have an initial bank account balance of 100 dollars. A given integer purchaseAmount represents the cost of a purchase.

Before the purchase is made, the purchaseAmount is rounded to the nearest multiple of 10. This rounded value is deducted from your balance, and the final balance is returned.

Examples

Example 1

Input: purchaseAmount = 9

Output: 90

The nearest multiple of 10 to 9 is 10. So your account balance becomes 100 - 10 = 90.

Example 2

Input: purchaseAmount = 15

Output: 80

The nearest multiple of 10 to 15 is 20. So your account balance becomes 100 - 20 = 80.

Example 3

Input: purchaseAmount = 10

Output: 90

10 is a multiple of 10 itself. So your account balance becomes 100 - 10 = 90.

Constraints

0 <= purchaseAmount <= 100

Solution Approach

Rounding Purchase Amount

To solve this, first round the purchaseAmount to the nearest multiple of 10. Use simple math to calculate this rounded value by either checking if the number is closer to a lower or higher multiple of 10.

Deduct from Account Balance

Once the purchase amount is rounded, subtract the rounded value from the starting balance of 100. This gives the new account balance after the purchase.

Edge Case Handling

Handle edge cases such as purchaseAmount being 0 or 100, where the result is straightforward. Ensure to test values at the rounding boundaries, like 5 or 95.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

Time complexity is O(1) because the operations consist only of rounding the number and performing a subtraction, both constant-time operations. The space complexity is O(1) as well since no additional data structures are required.

What Interviewers Usually Probe

Can the candidate clearly explain the rounding concept?
Does the candidate handle all edge cases correctly?
Is the candidate able to recognize the simplicity of the solution and not overcomplicate the problem?

Common Pitfalls or Variants

Common pitfalls

Failing to round correctly, especially for values like 5 or 95 that are at the rounding threshold.
Overcomplicating the solution by using unnecessary loops or conditional structures.
Not correctly handling the boundary cases such as 0 or 100.

Follow-up variants

Modify the problem to work with a different starting balance, such as 50 dollars, and adapt the solution accordingly.
Implement a similar solution where the rounding is done to a multiple of 5 instead of 10.
Extend the problem by adding a tax rate that must be deducted before rounding the purchase amount.

FAQ

How do I handle rounding the purchase amount to the nearest multiple of 10?

You can use simple math to round the purchase amount to the nearest multiple of 10 by checking the difference with the nearest multiples of 10.

What should I do when the `purchaseAmount` is exactly 5?

When the amount is 5, round it to 10 because 5 is closer to 10 than to 0.

What happens if the `purchaseAmount` is 0 or 100?

If the amount is 0, the final balance remains 100. If the amount is 100, the final balance becomes 0.

What is the time complexity of the solution?

The time complexity is O(1) because you only need to round the number and subtract from the initial balance.

How does GhostInterview help with this problem?

GhostInterview helps by clearly outlining the solution steps and ensuring that you understand the rounding logic and edge cases in this simple mathematical problem.

terminal

Solution

Solution 1: Enumeration + Simulation

We enumerate all multiples of 10 within the range $[0, 100]$, and find the one that is closest to `purchaseAmount`, denoted as $x$. The answer is $100 - x$.

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class Solution:
    def accountBalanceAfterPurchase(self, purchaseAmount: int) -> int:
        diff, x = 100, 0
        for y in range(100, -1, -10):
            if (t := abs(y - purchaseAmount)) < diff:
                diff = t
                x = y
        return 100 - x

Continue Practicing

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