Recursive shelves: First qualifying slip
Stuck or curious how this works? Visualize Binary Search ↗
Inputs
readings is a list of integers (numbers with no decimal part). It may be empty, and numbers may repeat unless task 1 says otherwise.
Your tasks
- Create a list
sectionSums. Start with the wholereadingslist as a section. Record the sum of that section. If it has more than one number, split it into a left half and a right half. Use the same steps on the left half first, then on the right half. When a section has an odd length, the left half has one fewer number. A section with one number is not split. Do not record empty sections.sectionSumscontains the sums in the order you record them. - Read
sectionSumsfrom left to right. Find the first number that is at leastkand store that number, not its position, infirstMatch. - Complete
recursiveShelvesFirstQualifyingSlipso it returnsfirstMatch. Return it, do not print it.
Example
If readings is [-1, 4, 8, 8, 4] and k is 8, then sectionSums is [23, 3, -1, 4, 20, 8, 12, 8, 4] and recursiveShelvesFirstQualifyingSlip returns 23.
Rules
- If no number is at least
k, return -1. k(a whole number from 1 to 8) is the threshold in task 2: compare each number with it.
Test data
TEST DATA 1: readings is [-4, 7, 15, 7, 0] and k is 1.
TEST DATA 2: readings is [-3, 8, 8, 8] and k is 4.
Examples
Input: readings = [-4,7,15,7,0] k = 1 Output: 25
sectionSums is [25, 3, -4, 7, 22, 15, 7, 7, 0]. Here k is 1. The first number at least 1 is 25. Return that value.
Input: readings = [-3,8,8,8] k = 4 Output: 21
sectionSums is [21, 5, -3, 8, 16, 8, 8]. Here k is 4. The first number at least 4 is 21. Return that value.
Input: readings = [] k = 8 Output: -1
sectionSums is []. Here k is 8. No number is at least 8, so return -1.
Input limits
- 0 <= input size <= 64.
- k is a whole number from 1 to 8. Answers fit in a signed 32-bit integer: -2,147,483,648 through 2,147,483,647.
- Input numbers have no decimal part. They range from -1,000,000 to 1,000,000. Follow any extra input rules in task 1.
Efficiency goal
Big-O describes how the work or memory needed grows as the input gets larger.
Efficiency goal: time O(n log n); extra memory, including the helper list, O(n). n is the input length. These labels describe how work and memory grow with the input.
Practice focus
Build sectionSums first, then return exactly what the tasks ask for. Follow the order, keep repeated values when the rule says so, and check the empty case.