Longest Peak

[drkennetz]

Longest Peak

Write a function that takes in an array of integers and returns the length of the longest peak in the array.

A peak is defined as adjacent integers in the array that are strictly increasing until they reach a tip (the highest value in the peak), at which point they become strictly decreasing. At least three integers are required to form a peak.

I will make this abundantly clear in the examples.

Business Rules/Errata

1. A peak must increase and then decrease.
2. The input can be an empty array, and must be an array of integers.
3. If equal value numbers appear in the run of a peak, the peak is stopped and restarted.
4. A peak must contain at least 3 numbers, such as 1,2,1
5. Numbers can be positive or negative.
6. Bonus: Optimal time and space: O(n) time | O(1) space - where n is the length of the input array.

Examples

longest_peak([5, 4, 3, 2, 1]) -> 0 # all decreasing
longest_peak([1, 2, 3, 4, 5]) -> 0 # all increasing
longest_peak([5, 4, 3, 2, 1, 2, 10, 12]) -> 0 # all decreasing, then all increasing
longest_peak([]) -> 0 # empty input check mandatory :D
longest_peak([1, 3, 2]) -> 3 # normal peak
longest_peak([1, 2, 3, 4, 5, 1]) -> 6 # peak ends on last number
longest_peak([2,2,3,2]) -> 3 # repeat at the beginning
longest_peak([1, 2, 3, 2, 1, 1]) -> 5 # repeat at the end
longest_peak([1, 2, 3, 3, 2, 1]) -> 0 # equality at peak tip, 0 is returned
longest_peak([1, 1, 1, 2, 3, 10, 12, -3, 2, 3, 45, 800, 99, 98, 0, -1, 2, 3, 4, 5, 0, -1]) -> 9 # peak in the middle -3, 2, 3, 45, 800, 99, 98, 0, -1

[mrumiker]

Cool problem. I noticed that the examples never tested a scenario where a peak is ended by an immediate uphill (you always had repeated numbers doing this job). Edited some of the plateaus out of the last example to remedy this.

[xanderyzwich]

It seems to be inferred but not directly said that the decline of the peak must be the same length as the incline. Is that correct?

[mrumiker]

@xanderyzwich Doesn't have to be - check out the [2, 2, 3, 2] example

[xanderyzwich]

@mrumiker That's a run of 3 [2, 3, 2] with rise and fall both being 1

[mrumiker]

@xanderyzwich You're right! From the instructions, I interpret a peak as any increasing run of numbers followed immediately by a decreasing run. Any "plateau" (a pair of equal numbers) would end the peak, as would any increasing pair occurring on the downhill portion.

[xanderyzwich]

So it seems like I'm wrong if this one is right

longest_peak([1, 2, 3, 4, 5, 1]) -> 6 # peak ends on last number

[xanderyzwich]

closed by #272

[mrumiker]

Sorry about the mixup on this - I realize my comment where I said "You're right!" wasn't clear. I was trying to say "You're right about the example I mentioned, but I still don't think the increase and decrease have to be equal based on what the directions say." I wish I'd actually noticed the example that proved my point!