Algorithm — Reverse Only Letters: A Python Solution

In many programming interviews, candidates encounter challenges that test their ability to manipulate strings efficiently. One such problem involves reversing a string with a twist: only the letters should be reversed, while non-letter characters remain in their original positions. In this blog post, we’ll explore this problem and present an optimized Python solution.

Problem Statement

Given a string s, we need to reverse it according to the following rules:

• All characters that are not English letters remain in the same position.
• All English letters (both lowercase and uppercase) should be reversed.

Initial Approach

Initially, one might consider creating a new string by iterating through the input string and reversing letters while keeping non-letter characters unchanged. However, this approach can be inefficient, especially for longer strings.

Optimized Solution

To optimize the solution, we can utilize a two-pointer approach. We’ll use pointers to traverse the string from both ends, swapping letters when both pointers are positioned on letters. Here’s the Python implementation:

class Solution:
    def reverseOnlyLetters(self, s: str) -> str:
        s = list(s)
        l, r = 0, len(s) - 1
        while l < r:
            if not s[l].isalpha():
                l += 1
            elif not s[r].isalpha():
                r -= 1
            else:
                s[l], s[r] = s[r], s[l]
                l += 1
                r -= 1
        return ''.join(s)

Explanation

• We start by converting the input string into a list to allow for in-place manipulation.
• Using two pointers (l and r), we iterate through the string from both ends towards the center.
• At each step, we check if both characters at the pointers are letters. If not, we increment/decrement the pointers accordingly.
• When both pointers point to letters, we swap them, effectively reversing the letters.
• We continue this process until the pointers meet or cross each other.

Time and Space Complexity

• Time Complexity: The time complexity of this solution is O(n), where n is the length of the input string. This is because we iterate through the string only once.
• Space Complexity: The space complexity is also O(n) due to the conversion of the string into a list. However, the algorithm operates in place without using any extra data structures except for the input string.

Conclusion

In this blog post, we explored an optimized solution to the problem of reversing a string with specific rules. By utilizing a two-pointer approach, we achieved a more efficient solution in terms of time and space complexity. This solution demonstrates the importance of algorithmic optimization in solving programming challenges effectively.

References

• LeetCode Problem: 917. Reverse Only Letters

Thank you for reading! I hope you found this blog post insightful and informative. If you have any questions or suggestions, feel free to leave a comment below. Don’t forget to like and share if you found this content helpful. Happy coding!