Problem A phrase is a palindrome if, after converting all uppercase letters to lowercase and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers. Given a string s, return true if it is a palindrome, or false otherwise.
Example Input s = A man, a plan, a canal: Panama. Output true. Explanation amanaplanacanalpanama is a palindrome.
Approach The key idea is to ignore all characters that are not letters or digits and compare the remaining characters in lowercase from both ends toward the center. A clean and readable way in C# is to use LINQ to filter and normalize the string, then apply two pointers for comparison. With LINQ, we remove non-alphanumeric characters and convert everything to lowercase. With two pointers, we traverse from the left and right and exit early if we find a difference; if we reach the center without differences, the string is a palindrome.
Code public bool IsPalindrome(string s) { var alphaNum = s.Where(char.IsLetterOrDigit).Select(char.ToLower).ToArray(); int left = 0, right = alphaNum.Length - 1; while (left < right) { if (alphaNum[left] != alphaNum[right]) return false; left++; right--; } return true; }
Notes This method is efficient in clarity and performance for the C# LeetCode 125 Valid Palindrome Easy problem. It avoids unnecessary manipulations and allows exiting as soon as a discrepancy is detected. Using LINQ improves readability by separating the cleaning phase from the two-pointer comparison phase.
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