Reverse String

Easy

📘 Problem Statement

Write a function that reverses a string. The input string is given as an array of characters. You must do this by modifying the input array in-place with \(O(1)\) extra memory.

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🔍 Example

Example 1:

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

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

🛠️ Solution

The problem imposes a constraint that you need to reverse the string in-place. This means you cannot use another array or routines such as reversed() or slicing like s[::-1] which actually creates a new list and does not do in-place reverse.

For in-place string reversal we will use Two Pointer approach, where we will have two pointers:

• left intially pointing to the start of the array and
• right which will be initiallized to point to the end of the array.

Now we will iterate throught the array and in each iteration we will :

• swap values of left and right
• increase left by 1 and decrease right by 1.
• continue to iterate as long as left < right.

def reverseString(s):
    left, right = 0, len(s) - 1

    while left < right:
        s[left], s[right] = s[right], s[left]
        left += 1
        right -= 1

🕒 Time Complexity: \(O(n)\)

📦 Space Complexity: \(O(1)\)