Track whether any adjacent pair violates increasing or decreasing order; the array is monotonic while at least one direction remains possible.
Complexity
O(n) time and O(1) space.
09 Sept 2026•Easy•Go / Python / TypeScript
Monotonic Array
Check whether an array is entirely nondecreasing or entirely nonincreasing.
Solution files
Go•monotonic-array/solution.go
package main
func isMonotonic(nums []int) bool {
nondecreasing, nonincreasing := true, true
for index := 1; index < len(nums); index++ {
if nums[index] < nums[index-1] {
nondecreasing = false
}
if nums[index] > nums[index-1] {
nonincreasing = false
}
}
return nondecreasing || nonincreasing
}
Python•monotonic-array/solution.py
class Solution:
def isMonotonic(self, nums: list[int]) -> bool:
nondecreasing = all(nums[index] >= nums[index - 1] for index in range(1, len(nums)))
nonincreasing = all(nums[index] <= nums[index - 1] for index in range(1, len(nums)))
return nondecreasing or nonincreasing
TypeScript•monotonic-array/solution.ts
function isMonotonic(nums: number[]): boolean {
let nondecreasing = true;
let nonincreasing = true;
for (let index = 1; index < nums.length; index += 1) { if (nums[index] < nums[index - 1]) nondecreasing = false; if (nums[index] > nums[index - 1]) nonincreasing = false; }
return nondecreasing || nonincreasing;
}