Reverse the Array

The problem

Reverse the given array in place so that the first element becomes the last and vice versa.

Every element must trade places with its mirror image across the center, and each swap fixes two positions at once. Marching two indices toward each other from the boundaries until they cross finishes the job in a single sweep without touching any extra memory.

Full solution

Two-pointer inward swap: one sweep from both ends, exactly n / 2 swaps, no extra memory. It is the in-place answer the statement asks for; the auxiliary-array copy spends O(n) space on something a swap does for free.

def reverse_array(arr: list[int]) -> list[int]:
    left, right = 0, len(arr) - 1
    # Stop when the pointers meet or cross; looping to n would swap everything twice.
    while left < right:
        arr[left], arr[right] = arr[right], arr[left]
        left += 1
        right -= 1
    return arr

function reverseArray(arr) {
  let left = 0;
  let right = arr.length - 1;
  // Stop when the pointers meet or cross; looping to n would swap everything twice.
  while (left < right) {
    [arr[left], arr[right]] = [arr[right], arr[left]];
    left++;
    right--;
  }
  return arr;
}

The theory behind it

Arrays — the ground this problem stands on. All Arrays problems

What Arrays is

An array is a row of fixed boxes laid side by side in computer memory, like numbered lockers in a hallway. Because each box occupies identical space and sits directly next to its neighbors, jumping to locker zero or locker ten thousand takes the exact same tiny fraction of time. Every box holds an item of the same type, addressed by an offset number called an index.

When to reach for it

Reach for an array when items arrive in a known sequence and need immediate retrieval by position number. Problems asking for running totals, prefix accumulations, cyclic rotations, or in-place rearrangements signal array mechanics. Whenever constraints require constant-time random lookups or contiguous cache scans across fixed collections, a flat sequence is the default container.

How the pattern works

Visualize a tape with zero-indexed slots stretching from start to end. Keep track of write and read cursors when modifying contents without allocating helper buffers. For running computations, maintain an invariant such as having processed all elements left of the current index while pending elements wait to the right. When modifying entries in place, consider scanning backwards from the end so unread data is not overwritten.

What each operation costs

Which roles need this problem

Arrays is a core topic for these 25 roles — if you're targeting one of them, this problem is early in your path, not optional.

Secondary for 3 more roles, including Database Engineer, Bioinformatics Engineer, Networking Engineer.

Companies that have asked it

Tags taken from the problem's own GeeksforGeeks page — not a copied list.