Interview question: Count Vowels

Given a string `s`, return the number of vowels (`a`, `e`, `i`, `o`, `u`, case-insensitive) it contains.

- Count the vowels `a`, `e`, `i`, `o`, and `u`.
- The check is case-insensitive, so uppercase vowels count too.
- Consonants, spaces, digits, and punctuation do not add to the total.

Understand What Counts

- To avoid checking both uppercase and lowercase forms, convert each character to lowercase.
- Then compare it with a small set of vowel characters.
- A set gives fast membership checks.

Normalize Characters

- Iterate through every character in the string.
- If the lowercase character is in the vowel set, increment the counter.
- The final counter is the answer.

Scan the String Once

- An empty string returns `0` because the loop never increments the count.
- Strings with no vowels also return `0`.
- Uppercase-only inputs work because each character is converted with `lower()`.

Handle Edge Cases Naturally

- The solution does one constant-time vowel check per character.
- Time complexity is O(n), where `n` is the length of the string.
- Space complexity is O(1) because the vowel set has a fixed size.