C# / .NET Junior (0–2 yrs) Coding Frequent C# two pointers strings palindrome LeetCode

Check if a string is a palindrome (ignoring case and non-alphanumeric characters)

cs-jun-007

Your answer

Answer as you would in a real interview — explain your thinking, not just the conclusion.

Model answer

I use two pointers starting at opposite ends and move inward, skipping non-alphanumeric characters using char.IsLetterOrDigit. At each position, compare the characters case-insensitively with char.ToLowerInvariant. If any pair mismatches, return false. If the pointers meet, return true. This is O(n) time and O(1) space — no string allocation needed, unlike normalising to a new string first.

Code example

public bool IsPalindrome(string s)
{
    int left = 0, right = s.Length - 1;
    while (left < right)
    {
        while (left < right && !char.IsLetterOrDigit(s[left]))  left++;
        while (left < right && !char.IsLetterOrDigit(s[right])) right--;
        if (char.ToLowerInvariant(s[left]) != char.ToLowerInvariant(s[right]))
            return false;
        left++; right--;
    }
    return true;
}

// Tests
Console.WriteLine(IsPalindrome("A man, a plan, a canal: Panama")); // true
Console.WriteLine(IsPalindrome("race a car"));                     // false

Follow-up

How would you check if an integer is a palindrome without converting it to a string?