Finds longest palindrome in a string in O(n*n) time and O(1) space.

LongestPalindrome.cs

private static string Expand(string str, int c1, int c2)
{
    int left = c1;
    int right = c2;

    while (left >= 0 && right < str.Length && str[left] == str[right])
    {
        left--;
        right++;
    }
    return str.Substring(left+1, right-left-1);
}

public static string LongestPalindrome(string str) 
{
    var chars = str.ToCharArray();
    var dp = new bool[chars.Length, chars.Length];
    string longest;
    
    for (int i = 0; i < str.Length-1; i++)
    {
        string str1 = Expand(str, i, i);
        if(str1.Length >= longest.Length)
        {
            longest = str1;
        }
        string str2 = Expand(str, i, i+1);
        if (str2.Length >= longest.Length)
        {
            longest = str2;
        }
    }

    return longest;
}