Reverse String

Write a function that reverses a list of characters. Do it in-place with O(1) extra memory — you may not allocate a second list.

Examples

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

Input:  ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]

Constraints

• 1 <= len(s) <= 10^5
• s[i] is a printable ASCII character.

Approach — two pointers

Put one pointer at the start and one at the end. Swap the two characters, then move the pointers toward each other. Stop when they meet in the middle. Each character is touched once, and swapping is a constant-time operation, so no extra memory is needed.

Solution

def reverse_string(s: list[str]) -> None:
    left, right = 0, len(s) - 1
    while left < right:
        s[left], s[right] = s[right], s[left]
        left += 1
        right -= 1

Explanation

• Why two pointers? Reversing is symmetric: the first element trades places with the last, the second with the second-to-last, and so on. Two converging pointers express exactly that pairing.
• Why left < right (not <=)? When they meet at the same index (odd-length list), that middle character stays put — swapping it with itself is pointless.
• In-place swap: Python's tuple assignment a, b = b, a swaps without a temp variable.

Complexity

• Time: O(n) — each character is visited once.
• Space: O(1) — only two integer indices, regardless of input size.

Interview tips

Interviewers use this to check you can reason about in-place mutation and pointer invariants. If asked to reverse an actual str (immutable in Python), note you'd return s[::-1] — but that allocates a new string, so it doesn't meet the O(1)-space bar.