Valid Palindrome
Given a string s, determine if it is a palindrome Consider only alphanumeric characters and ignore cases Example: "A man, a plan, a canal: Panama" => true Example: "race a car" => false
This is a two-pointers problem where the key is knowing which move can only improve things. It's a great warm-up that builds invariant-driven scanning without a ton of moving parts. Keep the invariant in your head and the pointer moves become obvious.
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Each locked part opens in the editor once the part before it passes.