Reverse Vowels of a String

The problem is to reverse only the vowels in a given string s. You can solve this problem by following a similar approach as reversing a whole string, but with some additional checks for vowels.

Here’s a step-by-step guide:

  1. Convert String to List: Since strings are immutable in Python, convert the string into a list of characters for in-place modifications.
  2. Use Two Pointers: Initialize two pointers, one at the beginning (left) and the other at the end (right) of the array.
  3. Find Vowels: Move the left pointer right and the right pointer left until they point to vowels.
  4. Swap the Vowels: Swap the characters at the left and right pointers if they are both vowels.
  5. Move the Pointers: Increment the left pointer and decrement the right pointer.
  6. Repeat Until Pointers Meet: Continue steps 3 to 5 until the left pointer is no longer less than the right pointer.
  7. Convert List to String: Convert the list back to a string and return.

Here’s a code example:

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class Solution:
    def reverseVowels(self, s: str) -> str:
        s = list(s)
        vowels = "aeiouAEIOU"
        left, right = 0, len(s) - 1

        while left < right:
            while left < right and s[left] not in vowels:
                left += 1
            while left < right and s[right] not in vowels:
                right -= 1

            s[left], s[right] = s[right], s[left]
            left += 1
            right -= 1

        return "".join(s)

Explanation:

  • The code starts by converting the string s into a list and initializing the left and right pointers.
  • Inside a loop, the pointers are moved until they point to vowels, at which point the vowels are swapped.
  • This continues until the pointers meet, and finally, the list is joined back into a string.

This solution respects the given constraints and reverses only the vowels in the string.

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class Solution:
    def reverseVowels(self, s: str) -> str:
        s = list(s)
        left = 0
        right = len(s) - 1
        m = 'aeiouAEIOU'

        while left < right:
            if s[left] in m and s[right] in m:
                s[left], s[right] = s[right], s[left]
                left += 1; right -= 1
            elif s[left] not in m:
                left += 1
            elif s[right] not in m:
                right -= 1

        return ''.join(s)

Problem Classification

This problem can be classified under the following categories:

  1. String Manipulation: This is the primary domain of this problem as it deals with modifying the input string. Specifically, the manipulation involves reversing certain characters (vowels) in the string.

  2. Two-pointer Technique: This technique could be inferred from the problem statement as we are to reverse certain elements (vowels) in the string, which can be efficiently done by maintaining two pointers pointing at the start and the end of the string and moving them inward.

  3. Character Classification: This problem involves classifying certain characters (vowels) as the target for specific operations. In this case, the operation is to reverse the vowels in the string.

  4. Case Sensitivity: The problem statement explicitly mentions that the vowels can be in either upper or lower case. This means that the solution needs to correctly handle both cases.

Language Agnostic Coding Drills

Here’s how we can break down this problem:

  1. String to List Conversion: The first step here is understanding how to convert a string to a list, as strings in many programming languages (including Python) are immutable, i.e., they cannot be changed once created. On the other hand, lists are mutable and can be changed in place.

  2. Character Classification (Vowels): You need to understand how to classify characters into certain groups (in this case, vowels). This will involve learning about membership checks, i.e., checking if a character belongs to a predefined group.

  3. Two-pointer Technique: Understanding how to use the two-pointer technique to traverse through an array or a list from both ends is crucial. It involves initializing two pointers at both ends and then iteratively moving them towards the center. This technique is especially useful when you have to do something with pairs of elements in the list, like swapping them in this case.

  4. Swapping Elements: The next step is to grasp the concept of swapping elements in an array. In Python, this can be done simply as a, b = b, a.

  5. Skipping Unnecessary Iterations: The logic to move the left pointer to the right when the left character is not a vowel and vice versa also needs to be understood and implemented correctly.

  6. Joining List Elements into a String: After all the vowel swapping is done, the final step is to convert the list back into a string. This involves understanding how to join elements of a list into a string.

The problem-solving approach for this problem can be broken down as follows:

  1. Convert the input string into a list for easy manipulation.
  2. Initialize two pointers, one at the start and the other at the end of the list.
  3. Check if the characters at the current left and right pointers are both vowels. If they are, swap them and move both pointers inwards.
  4. If the character at the left pointer is not a vowel, move the left pointer to the right.
  5. If the character at the right pointer is not a vowel, move the right pointer to the left.
  6. Repeat steps 3-5 until the left and right pointers cross each other.
  7. After all the vowels have been reversed, convert the list back into a string and return it as the final output.

Targeted Drills in Python

  1. String to List Conversion

    • Problem Statement: Write a function string_to_list that takes a string as an argument and returns a list of characters.
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    def string_to_list(s):
    return list(s)

  2. Character Classification (Vowels)

    • Problem Statement: Write a function is_vowel that takes a character and checks if it is a vowel.
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    def is_vowel(c):
    return c.lower() in 'aeiou'

  3. Two-pointer Technique

    • Problem Statement: Write a function sum_edges that takes a list of numbers and returns the sum of first and last elements. This is a simple implementation of the two-pointer technique.
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    def sum_edges(nums):
    if len(nums) == 0:
        return 0
    left, right = 0, len(nums) - 1
    return nums[left] + nums[right]

  4. Swapping Elements

    • Problem Statement: Write a function swap_elements that swaps the first and last elements of a list.
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    def swap_elements(nums):
    if len(nums) == 0:
        return nums
    nums[0], nums[-1] = nums[-1], nums[0]
    return nums

  5. Skipping Unnecessary Iterations

    • Problem Statement: Write a function skip_odd that skips the odd numbers in a list and returns only the even numbers.
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    def skip_odd(nums):
    return [num for num in nums if num % 2 == 0]

  6. Joining List Elements into a String

    • Problem Statement: Write a function list_to_string that takes a list of characters and returns a string.
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    def list_to_string(chars):
    return ''.join(chars)

You can practice these drills separately and then try to combine these skills to solve the given problem.