Reverse string

Introduction

reversing a string can be tricky in other programming languages. Let's see how easy and efficeint can this operation be in Python.

Problem

Write a function that reverses a list of characters. You have to manipulate the original string withoout any return statement.

Things you should know

Two pointers technique

Assume there is a variable, that indicates a position(index) in a list or an array.
You can call this variable a pointer, because this variable can be viewed as 'pointing' to a certain element in a list.
Obviously, in 'two pointers technique', there are two variables that stores positions of a list.
You can manipulate these variables for various tasks.

Swapping values of two variables

Instead of introducing a temporary variable, you can directly exchange values of multiple variables like this.

a = 1
b = 2

a, b = b, a

print(a) # 2
print(b) # 1

Solutions

1. Two pointers

from typing import List


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

2. Pythonic way

from typing import List


def reverse_string(s: List[str]) -> None:
    s.reverse()

References

• Leetcode Valid Palindrome Problem
• 파이썬 알고리즘 인터뷰(Python Algorithm Interview)