1095EasyArray

The Longest Run That Only Climbs or Only Falls

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Trains the technique from

LeetCode 3105Longest Strictly Increasing or Strictly Decreasing Subarray

This is an original problem, written from a brief that listed the technique, the difficulty, the topics, the function shape and the input bounds — none of that problem's wording, examples, hints or editorials. The link is there so you can map your practice onto the standard set.

Same function shape, different story and different numbers.

A strip of readings reads readings.

Return the length of the longest run of neighbouring readings that either climbs strictly all the way or falls strictly all the way. A run of a single reading counts as both.

Examples

Example 1

Input: readings = [1, 2, 5, 4]
Output: 3

The first three readings climb strictly, which is a run of three. The fall at the end reaches only two.

Example 2

Input: readings = [8, 8]
Output: 1

The two readings are equal, so neither climbs nor falls, leaving runs of one.

Example 3

Input: readings = [5, 4, 3, 4, 5, 6]
Output: 4

The strip falls for three readings and then climbs for four, so the climbing run is the longer.

Constraints

1 <= readings.length <= 50
1 <= readings[i] <= 50

The signature

The editor is preloaded with this. It matches the parent problem's shape, so a solution that works here transfers to a judge unchanged.

Python

def longest_monotonic_subarray(readings: list[int]) -> int:

Java

public int longestMonotonicSubarray(int[] readings)

The intended approach

Time O(n) over the strip's length.Space O(1).

Walk the strip once carrying two lengths: how long the strictly climbing run ending here is, and how long the strictly falling one is. Each reading extends whichever of the two its step continues and resets the other to one, since a single reading is a run of both kinds. Keep the largest of the two lengths seen anywhere. Equal neighbours break both runs at once, which the reset handles with no special case. One pass and three numbers.

Pythonentry point: longest_monotonic_subarray

def longest_monotonic_subarray(readings):
    best = 1
    up = 1
    down = 1
    for at in range(1, len(readings)):
        if readings[at] > readings[at - 1]:
            up += 1
            down = 1
        elif readings[at] < readings[at - 1]:
            down += 1
            up = 1
        else:
            up = 1
            down = 1
        if up > best:
            best = up
        if down > best:
            best = down
    return best

The obvious version

Try every run of the strip and test it outright: walk it once to see whether it climbs all the way and once to see whether it falls all the way, keeping the longest that passes either test. Costs the strip's length cubed rather than one pass.

This is not filler. It was written independently of the fast solution and the two were run against thousands of random inputs to confirm they agree — which is how the answer above is known to be right. Read it when the intended approach feels like a rabbit pulled out of a hat.

Python

def bf_longest_monotonic_subarray(readings):
    n = len(readings)
    best = 1
    for start in range(n):
        for end in range(start, n):
            climbs = True
            for at in range(start + 1, end + 1):
                if readings[at] <= readings[at - 1]:
                    climbs = False
                    break
            falls = True
            for at in range(start + 1, end + 1):
                if readings[at] >= readings[at - 1]:
                    falls = False
                    break
            if (climbs or falls) and end - start + 1 > best:
                best = end - start + 1
    return best

How this problem was checked

Reference solution executed against 15 test cases — 15 passed
3 worked examples confirmed by running the code
0 eight-word sequences shared with the parent problem's text
Agrees with an independent brute-force implementation on random inputs
Test set proven to catch deliberately broken solutions
Reviewed by a separate quality pass
Function arity matches the parent problem's

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