Monotonic Array
Easy · Arrays & Hashing · Array
Problem
An array is monotonic when it never changes direction: it is either entirely non-decreasing (nums[i] = nums[j] whenever i <= j). Equal neighbors are allowed either way.
Given an integer array nums, return true if it is monotonic and false otherwise.
Examples
Input: nums = [9,9,4,1] Output: true
Every step stays level or goes down, so the array is non-increasing.
Input: nums = [2,5,5,3] Output: false
The step from 2 to 5 goes up and the step from 5 to 3 goes down: two directions.
Input: nums = [-1] Output: true
A single value has no steps at all, so it is monotonic.
Constraints
- 1 <= len(nums) <= 10^5
- -10^5 <= nums[i] <= 10^5
Approaches
- Compare with the sorted copy — time O(n log n), space O(n)
Follow-up
What if the array arrives as a stream and you must answer after every new value? The two flags are all the state you need: update them with the newest step and report not (rises and falls). Once both are set the answer is false forever.
Related problems
Start free on Cariara · Coding problems · System design studio
