DSA interview question

Check if an Array is Sorted

Easy · Algorithms · Topics: array

The problem

Before running a binary search, a data pipeline must confirm that each incoming batch of IDs is sorted in non-decreasing order, because binary search silently returns wrong answers on unsorted data. The check itself is one pass: an array is sorted exactly when every element is at least as large as the one before it. A popular variant (LeetCode 1752) asks whether the array is sorted after some rotation, like [3, 4, 5, 1, 2], and it only takes one extra observation.

Example: [10, 20, 30, 40, 50] → true; [90, 80, 100, 70] → false.

Function signature

bool isSorted(const vector<int>& a);            // non-decreasing?
bool isSortedAndRotated(const vector<int>& a);  // non-decreasing after some rotation?

Test cases

Input `a`	`isSorted`	`isSortedAndRotated`	Explanation
`[10, 20, 30, 40, 50]`	true	true	Rotation by 0
`[90, 80, 100, 70, 40, 30]`	false	false	Several drops
`[1, 1, 2, 2]`	true	true	Equal neighbours are allowed
`[3, 4, 5, 1, 2]`	false	true	One drop (5 → 1), and the last element ≤ the first
`[2, 1, 3, 4]`	false	false	One drop, but 4 > 2 across the wrap
`[5]`	true	true
`[]`	true	true	Vacuously sorted

(Confirm: "sorted" means non-decreasing, so equal neighbours are fine.)

How interviewers phrase it

"Check whether an array is sorted" (GeeksforGeeks).
"Check if an array is sorted and rotated" (LeetCode 1752).
As a sanity check before binary search, or as part of "minimum swaps to sort" discussions.

Clarifying questions to ask

Non-decreasing (allowing equal neighbours) or strictly increasing?
Ascending only, or either direction?
For the rotated version: can the array contain duplicates?

Follow-ups to expect

"What's the condition?"
"How do you check sorted-and-rotated?"
"Why include the wrap-around pair?"
"Can you do it recursively?"
"Complexity?"

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