In-Place Reversal

The Idea

Swap the first and last elements, then step both indexes inward and repeat. After n/2 swaps the whole array is reversed. Time is O(n) and extra space is O(1).

Real-World Example

Think of reversing the play queue on your phone. The app cannot afford a second copy of a 5,000-song queue in memory, so it swaps the ends inward until the order flips.

The Code

def reverse(nums):
    left, right = 0, len(nums) - 1
    while left < right:
        # swap the two ends, then step inward
        nums[left], nums[right] = nums[right], nums[left]
        left += 1
        right -= 1
    return nums

print(reverse([1, 2, 3, 4, 5]))   # [5, 4, 3, 2, 1]
# n // 2 swaps, no second array -> O(n) time, O(1) space

a = [1, 2, 3, 4]
l, r = 0, 3
while l < r:
  a[l], a[r] = a[r], a[l]
  l += 1
  r -= 1
print(a)