ReverseString

Problem

https://leetcode.com/problems/reverse-string/

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

Example 1:

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

Example 2:

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

Constraints:

• 1 <= s.length <= 10:sup:`5`

• s[i] is a printable ascii character.

Solution

Let \(n\) be the length of the input list s. Let end = n - 1. We can reverse the list by swapping the outermost elements, moving inwards.

s[i], s[end - i] = s[end - i], s[i]

Pattern

Two Pointers, String

Code

def reverseString(s):
    """
    Do not return anything, modify s in-place instead.
    """
    end = len(s) - 1
    for i in range(len(s) // 2):
        s[i], s[end - i] = s[end - i], s[i]

Test

>>> from ReverseString import reverseString
>>> s = ['h', 'e', 'l', 'l', 'o']
>>> reverseString(s)
>>> s
['o', 'l', 'l', 'e', 'h']
>>> s = ['H', 'a', 'n', 'n', 'a', 'h']
>>> reverseString(s)
>>> s
['h', 'a', 'n', 'n', 'a', 'H']

ReverseString.reverseString(s)

Do not return anything, modify s in-place instead.