An isogram is a word or phrase where every letter appears at most once. The check is case-insensitive, meaning the letter O and the letter o are treated as the same letter. Non-letter characters are ignored during the check. Return true for an empty string or null input since there are no repeated letters.
Examples:
Input: str = 'Dermatoglyphics'
Output: true
Explanation: Every letter appears exactly once
Input: str = 'hello'
Output: false
Explanation: The letter l appears twice
Input: str = 'moOse'
Output: false
Explanation: Case-insensitive check, o appears twice (O and o)
Input: str = ''
Output: true
Explanation: Empty string has no repeated letters
Input: str = null
Output: true
Explanation: Null input returns true
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