DSA interview question

Left Rotate an Array by One

Easy · Algorithms · Topics: array

The problem

A digital signage screen cycles through a fixed list of adverts: every minute, the advert at the front moves to the back and everything else shifts forward one slot. Represented as an array, that's a left rotation by one: [A, B, C, D] becomes [B, C, D, A]. It has to happen in place, since the screen's controller has almost no spare memory. It's a tiny problem, but it sets up the "rotate by K" problem that follows and practises shifting elements without losing the one you overwrite first.

Example: [1, 2, 3, 4, 5] → [2, 3, 4, 5, 1].

Function signature

void leftRotateByOne(vector<int>& a);  // in place

Test cases

Input `a`	Output	Explanation
`[1, 2, 3, 4, 5]`	`[2, 3, 4, 5, 1]`	The first element moves to the end
`[1, 2]`	`[2, 1]`
`[3]`	`[3]`	Nothing changes
`[]`	`[]`	Nothing to rotate
`[9, 9, 1]`	`[9, 1, 9]`	Duplicates are fine

(Confirm: rotate in place with O(1) extra space.)

How interviewers phrase it

"Rotate an array to the left by one position."
"Cyclically rotate an array by one" (usually to the right).
A warm-up for "rotate by K positions".

Clarifying questions to ask

Left or right rotation?
In place, or can I return a new array?
Can the array be empty?

Follow-ups to expect

"Why save a[0] first?"
"How would you rotate right by one?"
"Is there a library function?"
"What if you had to rotate by k?"
"Complexity?"

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