Simple sorted list
Input
is_sorted([1, 2, 2, 3])
Output
True
Each item is less than or equal to the next, so the list is sorted.
Full lesson preview
Check if an array is sorted
Determine whether a list is sorted in non-decreasing order.
Python practice10 minSearching & SortingBeginnerLast updated March 25, 2026
Problem statement
Given a list of comparable values (numbers), determine whether the list is sorted in non-decreasing (ascending) order. Return True for empty lists and single-element lists. The function should run in O(n) time and O(1) extra space.
Task
Implement a function that returns True if the given list is sorted in non-decreasing order, otherwise False.
Examples
Input format
A single Python list: is_sorted(arr)
Output format
A boolean value (True or False) indicating whether arr is sorted in non-decreasing order.
Constraints
Do not use the built-in sorted() to compare; iterate once and compare neighbors. Works with ints and floats.
Samples
Sample 1
Input
is_sorted([])
Output
True
Empty list is considered sorted.