Create a function that reverses the vowels in a string but keeps the rest of the string the same.

Here’s a Python function that reverses only the vowels in a string while keeping the rest of the characters in place

def reverse_vowels(s):
    # Define vowels
    vowels = "aeiouAEIOU"
    
    # Convert the string to a list to modify it
    s_list = list(s)
    
    # Initialize two pointers
    left, right = 0, len(s) - 1
    
    # Traverse from both ends to find vowels and swap them
    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
        s_list[left], s_list[right] = s_list[right], s_list[left]
        
        # Move both pointers inward
        left += 1
        right -= 1
    
    # Join the list back into a string and return it
    return ''.join(s_list)

Table of Contents

Explanation:

Initialize Pointers: Use two pointers, one starting from the left (left) and one from the right (right).
Identify Vowels: Move each pointer until it finds a vowel.
Swap Vowels: Once both pointers are at vowels, swap them.
Continue Until Overlap: Repeat the process until left and right pointers cross each other.
Return Result: Convert the list back to a string and return it.

print(reverse_vowels("hello"))  # Output: "holle"
print(reverse_vowels("leetcode"))  # Output: "leotcede"

Here’s another explanation for the function:

Identify Vowels: We define vowels as the characters “a, e, i, o, u” (both uppercase and lowercase).
Convert String to List: Since strings in Python are immutable (they can’t be modified in place), we convert the string to a list of characters, s_list, so that we can swap vowels within it.
Use Two Pointers: We set two pointers—left at the start and right at the end of the list. The left pointer will move from the beginning, and the right pointer will move from the end of the string. Both pointers will work towards each other until they meet in the middle.
Move to Vowels:

The left pointer moves forward until it finds a vowel (if it’s not a vowel, it keeps moving right).
Similarly, the right pointer moves leftward until it finds a vowel.

Swap the Vowels: Once both pointers are at vowels, we swap the characters at left and right in s_list. This reverses the positions of the vowels without changing the position of any other characters.
Repeat Until Pointers Cross: After swapping, we move the left pointer one step right and the right pointer one step left. The loop continues until left and right cross each other, meaning all vowels have been reversed.
Join List into String: Finally, we join the list back into a single string and return it. The non-vowel characters stay in their original positions, while only the vowels are reversed.

Example Walkthrough

For the input "hello":

left finds the vowel 'e', and right finds the vowel 'o'.
These vowels are swapped, resulting in "holle".
After the swap, left and right pointers meet, and the final string with reversed vowels is returned: "holle".