## What is it? Check whether a string is a palindrome after ignoring spaces (and optionally case). A palindrome reads the same forwards and backwards. ## How it works - Use two pointers: `left = 0`, `right = n-1` - Skip spaces (and non-alphanumeric characters) by advancing the respective pointer - Compare `str[left]` with `str[right]` (case-insensitive if required) - If they differ → not a palindrome - If `left >= right` without a mismatch → it is a palindrome ## When to use - Validating phrases like "A man a plan a canal Panama" - Preprocessing strings before palindrome checks in larger problems - Interview problems involving string validation ## Key Points - Two-pointer approach is O(n) time and O(1) space - Handle edge cases: all spaces, single character, empty string - Extend easily to ignore punctuation and case for "valid palindrome" variants