patterncsharpModerate

Return the largest palindrome from the string

Submitted by: @import:stackexchange-codereview·Mar 10, 2026·

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thereturnlargestpalindromefromstring

Problem

Here is the question: find the largest palindrome from a string.

Ex:

ABCBAHELLOHOWRACECARAREYOUILOVEUEVOLIIAMAIDOINGGOOD

Result:

ILOVEUEVOLI

I am not sure of the efficiency of the algorithm.

static void Main(string[] args)
{
    var str = "ABCBAHELLOHOWRACECARAREYOUILOVEUEVOLIIAMAIDOINGGOOD";
    var longestPalindrome = GetLongestPalindrome(str);
    Console.WriteLine(longestPalindrome);
    Console.Read();
}

private static string GetLongestPalindrome(string input)
{
    int rightIndex = 0, leftIndex = 0;
    List paliList = new List();
    string currentPalindrome = string.Empty;
    string longestPalindrome = string.Empty;
    for (int currentIndex = 1; currentIndex = 0 && rightIndex  w.Length).First();
    return x; 
}

Solution

Your basic algorithm seems pretty efficient, but building a list then sorting it just to find the longest one isn't very efficient. It would be much more efficient to just keep track of the longest palindrome:

private static string GetLongestPalindrome(string input)
    {
        int rightIndex = 0, leftIndex = 0;
        var x = "";
        string currentPalindrome = string.Empty;
        string longestPalindrome = string.Empty;
        for(int currentIndex = 1; currentIndex = 0 && rightIndex  x.Length)
                    x = currentPalindrome;
                leftIndex--;
                rightIndex++;
            }
        }
        return x;
    }

In my tests this is 3 times faster.

I hate to break this to you, but there is a bug in your code, which will probably mean rewriting your algorithm. You algorithm assumes that the palindrome will be an odd number of characters. However, a palindrome can be an even number of characters and your code won't find it if it is.

Here's some code that will find the longest palindrome regardless:

static string LargestPalindrome(string input)
{
    string output = "";
    int minimum = 2;
    for(int i = 0; i  minimum)
            {
                output = forstr;
                minimum = forstr.Length;
            }
        }
    }
    return output;
}

EDIT

The above code has a bug. Here it is reworked:

static string LargestPalindrome(string input)
{
    int longest = 0;
    int limit = input.Length;
    for (int i = 0; i  i; j--)
        {
            string forStr = input.Substring(i, j - i);
            string revStr = new string(forStr.Reverse().ToArray());
            if (forStr == revStr && forStr.Length > longest)
            {
                return forStr;
            }
        }
    }
    return "";
}

Code Snippets

private static string GetLongestPalindrome(string input)
    {
        int rightIndex = 0, leftIndex = 0;
        var x = "";
        string currentPalindrome = string.Empty;
        string longestPalindrome = string.Empty;
        for(int currentIndex = 1; currentIndex < input.Length - 1; currentIndex++)
        {
            leftIndex = currentIndex - 1;
            rightIndex = currentIndex + 1;
            while(leftIndex >= 0 && rightIndex < input.Length)
            {
                if(input[leftIndex] != input[rightIndex])
                {
                    break;
                }
                currentPalindrome = input.Substring(leftIndex, rightIndex - leftIndex + 1);
                if(currentPalindrome.Length > x.Length)
                    x = currentPalindrome;
                leftIndex--;
                rightIndex++;
            }
        }
        return x;
    }

static string LargestPalindrome(string input)
{
    string output = "";
    int minimum = 2;
    for(int i = 0; i < input.Length - minimum; i++)
    {
        for(int j = i + minimum; j < input.Length - minimum; j++)
        {
            string forstr = input.Substring(i, j - i);
            string revstr = new string(forstr.Reverse().ToArray());
            if(forstr == revstr && forstr.Length > minimum)
            {
                output = forstr;
                minimum = forstr.Length;
            }
        }
    }
    return output;
}

static string LargestPalindrome(string input)
{
    int longest = 0;
    int limit = input.Length;
    for (int i = 0; i < limit; i++)
    {
        for (int j = limit-1; j > i; j--)
        {
            string forStr = input.Substring(i, j - i);
            string revStr = new string(forStr.Reverse().ToArray());
            if (forStr == revStr && forStr.Length > longest)
            {
                return forStr;
            }
        }
    }
    return "";
}

Context

StackExchange Code Review Q#43571, answer score: 11

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