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Count the Number of Vowel Strings in Range

Number: 2654

Difficulty: Easy

Paid? No

Companies: PayPal

Problem Description

Given a 0-indexed array of strings called words, and two integers left and right, count how many strings within the range [left, right] are "vowel strings". A vowel string is defined as a string that starts and ends with a vowel (vowels are 'a', 'e', 'i', 'o', 'u').

Key Insights

  • Iterate only over the subarray from index left to right.
  • Check the first and last character of each string to determine if both are vowels.
  • Use a set to quickly check if a character is a vowel.
  • Maintain a count of valid strings and return it.

Space and Time Complexity

Time Complexity: O(n) where n is the number of strings in the range [left, right].
Space Complexity: O(1) since only a few auxiliary variables are used.

Solution

We approach the problem by iterating over the subarray of words from index left to right. For each word, we check if its first and last characters are vowels. A set containing vowels is used to verify these conditions efficiently. If both conditions hold true, we increment our count. This algorithm leverages a linear scan over the subarray, leading to a simple and efficient solution.

Code Solutions

# Define a function to count vowel strings in the given range.
def count_vowel_strings(words, left, right):
    # Set of vowels for quick lookup.
    vowels = {'a', 'e', 'i', 'o', 'u'}
    
    # Initialize a counter for valid vowel strings.
    count = 0
    
    # Loop through the specified range.
    for i in range(left, right + 1):
        word = words[i]
        # Check if both the first and last characters are vowels.
        if word[0] in vowels and word[-1] in vowels:
            count += 1
    return count

# Example usage:
words = ["are", "amy", "u"]
left, right = 0, 2
print(count_vowel_strings(words, left, right))  # Expected output: 2
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