Palindrome number product of two 3-digit numbers - Project Euler Solution 4 - Brute Force not working

Question

I am using this code to solve problem. 4 on Project Euler. The problem reads:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
Find the largest palindrome made from the product of two 3-digit numbers.

I have tried to solve the problem. My code returns a palindrome number and a fairly big one too. But it is incorrect. I have also found a correct solution at MathBlog.dk. But what is wrong with my code? --

long result = 0;
for (long x = 100; x <= 999; x++)
{
    for (long y = 100; y <= 999; y++)
    {
        long num = x * y;
        if (num.ToString() == StringFunctions.ReverseString(num.ToString()))
        {
                result = num;
        }
    }
}
Console.WriteLine("Result : {0}", result);

The StringFunctions.RevereString function is as follows:

public static class StringFunctions
{
    public static string ReverseString(string s)
    {
        string result = "";
        for (int i = s.Length - 1; i >= 0; i--)
        {
            result += s.Substring(i, 1);
        }
        return result;
    }
}

My code returns 580085 but the correct answer is 906609. I don't want to know about better solutions. I just want to know why my code isn't working.

Any help will be much appreciated. Thanks in advance.

You never check whether the newly calculated result is greater than than the current result. It may be that you're overwriting your highest value result with a lower value in your for loops. — Robert Lacher, Oct 23 '15 at 11:44
You are returning the **last** palindrome found, not the **biggest** one. — Dennis_E, Oct 23 '15 at 11:44
@rob and Dennis_E How could I even have missed that? Thanks for your help. — Fᴀʀʜᴀɴ Aɴᴀᴍ, Oct 23 '15 at 11:46
@Dennis_E that's because I have already found too many just by googling "Project Euler Solution to Problem 4". They solve the problem in ~0-1 ms. If you have an even better solution, I welcome you to let me know of it. — Fᴀʀʜᴀɴ Aɴᴀᴍ, Oct 23 '15 at 11:48
@FarhanAnam That makes sense. Solutions of the older problems can be found anywhere. — Dennis_E, Oct 23 '15 at 11:54
a faster way would be to start from 999 to 100. and the condintion `if (num > max && num.ToString() == num.Reversed)` will not check to reverse if num is smaller than biggest found number. — M.kazem Akhgary, Oct 23 '15 at 11:55

score 2 · Accepted Answer · edited Oct 23 '15 at 17:03

Your result is storing the last palindrome number obtained from the loop but not the largest one.

Both the variables X and Y iterate from 100 to 999

Imagine a case when (assuming all obtained numbers are palindromes) x = 500 and y = 500. It will execute earlier than x = 990 and y = 100. But in the earlier case the palindrome is larger but your code stores the smaller one. Use an if condition to get the largest palindrome:

long result = 0;
for (long x = 100; x <= 999; x++)
{
    for (long y = 100; y <= 999; y++)
    {
        long num = x * y;
        if (num.ToString() == StringFunctions.ReverseString(num.ToString()))
        {
            if(result < num)
            {
            result = num;
            }
        }
    }
}
Console.WriteLine("Result : {0}", result);

Palindrome number product of two 3-digit numbers - Project Euler Solution 4 - Brute Force not working

1 Answers1