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Reverse a Character Array In Place with Constant Extra Memory

SnowflakeSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

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Reverse a string given as an array of characters in place, using only constant extra memory, and return the same list. A short coding warm-up that tests index handling, swapping elements and odd and even lengths without building a copy.

Reverse a string given as an array of its characters. The reversal must happen in place: rearrange the elements of the given list itself, using only a constant amount of extra memory, and return that list.

Function Signature

def reverse_characters(chars: list[str]) -> list[str]:

Rules

  • Modify chars itself. Do not build a reversed copy, and do not call a built-in reverse routine or use slicing that creates a new list.
  • Return the same list object after the reversal.

Constraints

  • 1 <= len(chars) <= 10^5
  • every element is a single printable ASCII character (codes 32 to 126); spaces and punctuation are ordinary characters

Examples

Example 1

Input: chars = ["h", "e", "l", "l", "o"]
Output: ["o", "l", "l", "e", "h"]

Example 2

Input: chars = ["a", " ", "b", "!"]
Output: ["!", "b", " ", "a"]

With an even length, every element moves; the space is reversed like any other character.

Example 3

Input: chars = ["z"]
Output: ["z"]