Valid Palindrome

Problem: A phrase is a palindrome if, after converting all uppercase letters into lowercase letters 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.

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.

Approach: It's a simple two pointer problem. You can understand it by looking at the implementation.

Implementation in C#:

    public bool IsPalindrome(string s)

    {

        int length = s?.Length ?? 0;

        if (length <= 1)

        {

            return true;

        }

        int start = 0, end = length - 1;

        while (start < end)

        {

            while (start < end && !Char.IsLetterOrDigit(s[start]))

            {

                ++start;

            }

            while (start < end && !Char.IsLetterOrDigit(s[end]))

            {

                --end;

            }

            if (start == end)

            {

                break;

            }

            if (Char.ToLower(s[start]) != Char.ToLower(s[end]))

            {

                return false;

            }

            ++start;

            --end;

        }

        return true;

    }

Complexity: O(n)