LeetCode 896: Monotonic Array ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2022-09-24

Submissions: Wrong Answer ×3 → Accepted ×3

Approach
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Monotonic has two directions and is non-strict (equal neighbors are fine). In one pass record whether any rise and any fall occur; they cannot both happen (Array Template 1).

My Attempts
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The first two checked only one direction: allowing only non-increasing made [0, 3] return False, and only non-decreasing made [1, 0] return False. Attempt 3 checked both but with strict < and >: [0, 0] returned False. Switching to <= and >= passed.

Solution
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class Solution:
    def isMonotonic(self, nums: List[int]) -> bool:
        up = down = False
        for i in range(1, len(nums)):
            if nums[i] > nums[i - 1]:
                up = True
            elif nums[i] < nums[i - 1]:
                down = True
        return not (up and down)

Complexity
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Time O(n), space O(1).

Mistakes to Avoid
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Consider both directions.
Use <= and >=; equal neighbors keep it monotonic.

Interview Explanation
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In one pass I record whether a rise and a fall occur; the array is monotonic unless both do, and equal neighbors do not matter. Time O(n), space O(1).