0567EasyArray

One-Way Altitude Log

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Trains the technique from

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 survey drone logs its altitude relative to sea level once a second, giving the array nums. Altitudes may be negative when the drone flies below sea level.

The flight is called one-way when the drone never reverses its vertical direction: either every reading is greater than or equal to the reading before it, or every reading is less than or equal to the reading before it. Holding altitude is not a reversal, so a run of equal readings is fine either way, and a log with a single reading is one-way.

Return true when the log is one-way and false otherwise.

Examples

Example 1

Input: nums = [3, 3, 5, 9, 9]
Output: true

Each reading is at least as high as the one before it, so the drone never descends.

Example 2

Input: nums = [1, 2, 2, 1]
Output: false

The drone climbs from 1 to 2 and later descends from 2 to 1, so the log holds both a rise and a fall.

Example 3

Input: nums = [2, 2, 1]
Output: true

The drone holds 2 and then descends, and holding is not a reversal, so every reading is at most the reading before it.

Constraints

1 <= nums.length <= 10^5
-10^5 <= nums[i] <= 10^5

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 is_monotonic(nums: list[int]) -> bool:

Java

public boolean isMonotonic(int[] nums)

The intended approach

Time O(n).Space O(1).

Carry two flags through a single walk over consecutive readings: one saying the log has so far never dropped, one saying it has so far never climbed. A step up clears the second, a step down clears the first, and equal readings clear neither. The log is one-way when at least one flag is still standing at the end.

Pythonentry point: is_monotonic

def is_monotonic(nums):
    # O(n) time, O(1) space: one pass carrying two flags, one for each direction;
    # a log is one-way when at least one flag survives the whole walk.
    climbs = True
    drops = True
    for i in range(1, len(nums)):
        if nums[i] > nums[i - 1]:
            drops = False
        elif nums[i] < nums[i - 1]:
            climbs = False
    return climbs or drops

The obvious version

Test every pair of positions twice over: once to see whether an earlier reading ever exceeds a later one, and once for the opposite, then accept when one of the two sweeps found no offending pair.

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_is_monotonic(nums):
    n = len(nums)
    up = True
    for i in range(n):
        for j in range(i + 1, n):
            if nums[i] > nums[j]:
                up = False
    down = True
    for i in range(n):
        for j in range(i + 1, n):
            if nums[i] < nums[j]:
                down = False
    return up or down

How this problem was checked

Reference solution executed against 13 test cases — 13 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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