Weekly Challenge: Peak Visitors

Weekly Challenge 345

Each week Mohammad S. Anwar sends out The Weekly Challenge, a chance for all of us to come up with solutions to two weekly tasks. My solutions are written in Python first, and then converted to Perl. It's a great way for us all to practice some coding.

Challenge, My solutions

Task 1: Peak Positions

Task

You are given an array of integers, @ints.

Find all the peaks in the array, a peak is an element that is strictly greater than its left and right neighbours. Return the indices of all such peak positions.

My solution

This is relatively straight forward. I start by comparing the first two elements and add 0 to the peaks list (array in Perl) if the first value is higher.

I then compare the middle values, adding the position to the peaks list if it's before and after values are lower. Finally I compare the last two values.

def peak_positions(ints: list[int]) -> list[int]:
    peaks = []

    if ints[0] > ints[1]:
        peaks.append(0)

    for pos in range(1, len(ints) - 1):
        if ints[pos] > ints[pos - 1] and ints[pos] > ints[pos + 1]:
            peaks.append(pos)

    if ints[-1] > ints[-2]:
        peaks.append(len(ints) - 1)

    return peaks

Examples

$ ./ch-1.py 1 3 2
(1)

$ ./ch-1.py 2 4 6 5 3
(2)

$ ./ch-1.py 1 2 3 2 4 1
(2, 4)

$ ./ch-1.py 5 3 1
(0)

$ ./ch-1.py 1 5 1 5 1 5 1
(1, 3, 5)

Task 2: Last Visitor

Task

You are given an integer array @ints where each element is either a positive integer or -1.

We process the array from left to right while maintaining two lists:

1. @seen stores previously seen positive integers (newest at the front)
2. @ans stores the answers for each -1

Rules:

1. If $ints[i] is a positive number -> insert it at the front of @seen
2. If $ints[i] is -1:
   1. Let $x be how many -1s in a row we’ve seen before this one.
   2. If $x < len(@seen) -> append seen[x] to @ans
   3. Else -> append -1 to @ans

At the end, return @ans.

My solution

I actually don't understand this task. However, I've followed the instructions in the task, and get the expected results. I renamed x as neg_count to make it more meaningful.

def last_visitor(ints: list[int]) -> list[int]:
    seen = []
    ans = []
    neg_count = 0

    for i in ints:
        if i > 0:
            seen.insert(0, i)
            neg_count = 0
        elif i == -1:
            if neg_count < len(seen):
                ans.append(seen[neg_count])
            else:
                ans.append(-1)
            neg_count += 1
        else:
            raise ValueError("Input integers must be positive or -1.")

    return ans

Examples

$ ./ch-2.py 5 -1 -1
(5, -1)

$ ./ch-2.py 3 7 -1 -1 -1
(7, 3, -1)

$ ./ch-2.py 2 -1 4 -1 -1 
(2, 4, 2)

$ ./ch-2.py 10 20 -1 30 -1 -1
(20, 30, 20)

$ ./ch-2.py -1 -1 5 -1
(-1, -1, 5)