Please use spoiler flag next time.

This is a stack data structure.

IMHO this is the best solution:

Explanation:

Edge Case Handling: Checks for edge cases where the input is invalid.

Length Calculation: Create an array lengths that stores the length of each string in strarr.

Initial Sum Calculation: Calculate the sum of the first k lengths and initialize maxSum and maxIndex.

Sliding Window Approach: Iterate through the array using a sliding window technique:

• Update currentSum by subtracting the length of the string that is sliding out of the window and adding the length of the string that is sliding into the window.
• Update maxSum and maxIndex if currentSum exceeds maxSum.

Construct the Result String: Use slice to get the k consecutive strings starting from maxIndex and concatenate them using join.

Efficiency Analysis:

Time Complexity: O(n), where n is the length of strarr. The solution involves a single pass to calculate lengths and a sliding window to find the maximum sum.
Space Complexity: O(n), due to the array lengths storing the lengths of the strings.

This solution is efficient and avoids unnecessary string concatenation, focusing on computing the lengths instead. This should be much faster, especially for large input sizes.

I believe the only parts missing are the definitions of constants for the cost of a day and the various discounts. Perhaps also consider using the ternary operator in the discountRate function.

I think that +v ?? v would be more accurate.

Looks very good but it isn't very efficient I think. So, I don't think it should be considered a best practice code.

Does TCO mean Time Complexity Optimal?