Problem

Given an integer array arr of size n, rotate it left by one position in place: the first element moves to the end and every other element shifts one index to the left.

Input / Output

• Input: integer array arr.
• Output: the rotated array.

Examples

Example 1 Input: arr = [1, 2, 3, 4, 5] Output: [2, 3, 4, 5, 1]

Example 2 Input: arr = [7] Output: [7]

Example 3 Input: arr = [4, 9] Output: [9, 4]

Constraints

• 1 <= n <= 10^5

Approach

1. Save first = arr[0].
2. For i from 1 to n-1, set arr[i-1] = arr[i].
3. Set arr[n-1] = first.

Each element is moved exactly once, so the cost is linear with constant extra memory. Building a new array arr[1:] + arr[:1] is simpler but uses O(n) additional space, which the interviewer will point out.

This problem is the warm-up for rotate by k, which generalises the idea: either repeat the single rotation k times (O(n·k), bad), or use the three-reversal trick (reverse the first k, reverse the rest, reverse everything) for O(n) time and O(1) space.

Complexity

• Time: O(n)
• Space: O(1)

Reference solution (Python)

def left_rotate_by_one(arr):
    if len(arr) <= 1:
        return arr
    first = arr[0]
    for i in range(1, len(arr)):
        arr[i - 1] = arr[i]
    arr[-1] = first
    return arr

Test cases

Input	Expected
[1,2,3,4,5]	[2,3,4,5,1]
[7]	[7]
[4,9]	[9,4]
[1,1,2]	[1,2,1]