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Reverse Vowels of a String

Difficulty: Easy

Problem Description

Given a string s, reverse only all the vowels in the string and return it. The vowels are 'a', 'e', 'i', 'o', and 'u', and they can appear in both lower and upper cases, more than once.

Key Insights

  • Identify the vowels in the string.
  • Use a two-pointer approach to reverse the positions of the vowels.
  • Ensure that non-vowel characters remain in their original positions.

Space and Time Complexity

Time Complexity: O(n) - where n is the length of the string. Space Complexity: O(n) - for storing the vowels, but can be considered O(1) if using in-place swapping.

Solution

The problem can be solved using a two-pointer technique. We will start with two pointers, one at the beginning and the other at the end of the string. We will move the pointers towards each other, swapping the vowels found at those positions. This continues until the two pointers meet.

The data structure used is a list to hold characters for easy manipulation, as strings in most programming languages are immutable.

Code Solutions

def reverseVowels(s: str) -> str:
    vowels = set('aeiouAEIOU')  # Set of vowels for quick lookup
    s_list = list(s)  # Convert string to list for mutability
    left, right = 0, len(s) - 1  # Initialize two pointers

    while left < right:
        # Move the left pointer to the next vowel
        while left < right and s_list[left] not in vowels:
            left += 1
        # Move the right pointer to the previous vowel
        while left < right and s_list[right] not in vowels:
            right -= 1
        # Swap the vowels found at the left and right pointers
        if left < right:
            s_list[left], s_list[right] = s_list[right], s_list[left]
            left += 1
            right -= 1

    return ''.join(s_list)  # Convert list back to string
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