Valid Palindrome
Match the edges. Keep the middle honest.
The challenge, in plain language +
A display label may contain letters, digits, spaces, and punctuation. Decide whether its letters and digits read the same from either end when letter case is ignored. Return a boolean. For this exercise, inputs contain ASCII characters, and an empty meaningful sequence counts as a palindrome.
Which wording most directly suggests two inward-moving pointers?
This lesson uses 2D: exact positions, widths and comparisons are easier to read in one plane.
Why this approach works
Every useful character outside the current left-to-right interval already belongs to a verified matching pair. The original answer is therefore the answer for the interval that remains.
Even though skipping uses inner loops, each pointer crosses at most n positions during the entire run. Total work is O(n); the pointers and single-character comparisons use O(1) extra space.
Common failure: Skipping punctuation on only one side. Either head may land on ignored characters, so both sides need their own skip step.
A fresh solve is the test.
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