Leetcode offers a plethora of problems that test a programmer’s ability to manipulate strings. Among these, the “Reverse Only Letters” problem is unique because it demands both string manipulation and character classification skills. This article will provide a detailed breakdown of the problem, the underlying logic, and Python-based solutions, followed by a complexity analysis.
Table of Contents:
- Understanding the Problem Statement
- Problem Analysis
- Two-Pointer Technique
- Python Implementation
- Complexity Analysis
- Alternate Approaches
1. Understanding the Problem Statement
Given a string
S, return the “reversed” string where all characters that are not a letter stay in the same place, and all letters reverse their positions.
Input: "ab-cd" Output: "dc-ba"
2. Problem Analysis
At a first glance, it might seem like we just need to reverse the string. However, the challenge is to ensure that non-letter characters remain in their original positions. We need a strategy to skip these characters while performing the reversal.
3. Two-Pointer Technique
An intuitive way to approach this problem is by using the two-pointer technique. We can have two pointers:
left: starts from the beginning of the string
right: starts from the end of the string
left is less than
right, we can perform the following operations:
- If the character at
leftis not a letter, increment
- If the character at
rightis not a letter, decrement
- If both characters are letters, swap them. Then, increment
This approach ensures that we only reverse the positions of letter characters.
4. Python Implementation
Here’s a Python function implementing the two-pointer technique for this problem:
def reverseOnlyLetters(S: str) -> str: S = list(S) # Convert string to list for mutable operations left, right = 0, len(S) - 1 while left < right: # If character at left pointer is not a letter, move to next character while left < right and not S[left].isalpha(): left += 1 # If character at right pointer is not a letter, move to previous character while left < right and not S[right].isalpha(): right -= 1 # Swap the letters at left and right pointers S[left], S[right] = S[right], S[left] left, right = left + 1, right - 1 return ''.join(S)
5. Complexity Analysis
Time Complexity: O(n)
The algorithm iterates over the string with the two pointers,
right. In the worst case, we go through the string once, leading to a linear time complexity.
Space Complexity: O(n)
We convert the input string to a list to easily swap characters. This conversion requires O(n) space.
6. Alternate Approaches
The two-pointer technique is efficient, but there might be other ways to tackle the problem:
- Stack: Iterate through the string and push only the letters onto a stack. Then, iterate through the string again. If the character is a letter, pop from the stack and replace it. Else, move to the next character.
- Regular Expressions: Using Python’s
remodule, one could extract all letters, reverse them, and then replace them back in the string.
The “Reverse Only Letters” problem is a classic example of how string manipulation can be combined with character classification in programming challenges. While the problem appears simple on the surface, it necessitates a clear understanding of string operations and character properties.