Valid Palindrome: the idea to remember.

The core insight

Skip ignored characters at both ends and compare the remaining pair.

Why it works

Every compared outer pair matches. Removing it preserves the palindrome question for the remaining interval.

The mistake to avoid

Check bounds while skipping punctuation, including strings consisting only of punctuation.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

boolean isPalindrome(String s) {
    int left = 0, right = s.length() - 1;
    while (left < right) {
        while (left < right && !Character.isLetterOrDigit(s.charAt(left))) left++;
        while (left < right && !Character.isLetterOrDigit(s.charAt(right))) right--;
        if (Character.toLowerCase(s.charAt(left)) !=
            Character.toLowerCase(s.charAt(right))) return false;
        left++;
        right--;
    }
    return true;
}

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