Longest Palindromic Substring in C#

Longest Palindromic

Substring in C#

Welcome to the Longest Palindromic Substring in C# page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

if (args is not { Length: > 0 })
    return Usage();

string result = LongestPalindrome(string.Join(' ', args));

if (result.Length < 2)
    return Usage();

Console.WriteLine(result);
return 0;

static string LongestPalindrome(string input)
{
    int length = input.Length;
    if (length < 2)
        return "";

    char[] transformed = new char[2 * length + 3];
    int[] radius = new int[transformed.Length];

    int index = 0;
    transformed[index++] = '^';

    ReadOnlySpan<char> source = input;

    foreach (char c in source)
    {
        transformed[index++] = '#';
        transformed[index++] = c;
    }

    transformed[index++] = '#';
    transformed[index++] = '$';

    int center = 0;
    int rightBoundary = 0;

    int bestCenter = 0;
    int bestRadius = 0;

    for (int i = 1; i < index - 1; i++)
    {
        int mirror = 2 * center - i;
        int R = radius[i];

        if (i < rightBoundary)
            R = Math.Min(rightBoundary - i, radius[mirror]);

        while (transformed[i + R + 1] == transformed[i - R - 1])
            R++;

        radius[i] = R;
        int expandedRight = i + R;

        center = expandedRight > rightBoundary ? i : center;
        rightBoundary = Math.Max(expandedRight, rightBoundary);

        bool isBest = R > bestRadius;
        bestRadius = isBest ? R : bestRadius;
        bestCenter = isBest ? i : bestCenter;
    }

    if (bestRadius < 2)
        return "";

    int startIndex = (bestCenter - bestRadius) / 2;
    return input.AsSpan(startIndex, bestRadius).ToString();
}

static int Usage()
{
    Console.Error.WriteLine("Usage: please provide a string that contains at least one palindrome");
    return 1;
}

Longest Palindromic Substring in C# was written by:

• Maxwell Maslov
• Ștefan-Iulian Alecu

If you see anything you'd like to change or update, please consider contributing.

How to Implement the Solution

No 'How to Implement the Solution' section available. Please consider contributing.

How to Run the Solution

No 'How to Run the Solution' section available. Please consider contributing.