How to find if a number is a Palindrome?

Read the question. Listen very carefully to the question. Each word may have a meaning. In my experience, I have seen many interviewees just hear the last word: palindrome. Then I start getting solutions and examples of palindrome words. But note the question: It says a number. String operations are way more costly than number crunching.

Let's now dive into some possible solutions for this problem.

public static bool IsPalindrome(long number)
{
bool isPalin = true;

// inefficient solution - convert number to string
string numAsStr = number.ToString();

// more inefficiency - looping through the entire string
// could have achieved the same by traversing just half the string
for (int i = 0, j = numAsStr.Length - 1; i < numAsStr.Length - 1; i++, j--)
{
if (numAsStr[i] != numAsStr[j])
{
// the corresponding locations do not match
isPalin = false;
// some people forget to break out of the loop
// extremely bad - no need to do any further comparisons
break;
}
}
return isPalin;
}

The above solution was an in-efficient solution. It works, but does not get you any points. So, now let's see what improvements can be made to the above program.

public static bool IsPalindrome2(long number)
{
bool isPalin = true;

// inefficient solution - convert number to string
string numAsStr = number.ToString();

// we can find if a string is palindrome or not by just traversing half the string
for (int i = 0, j = numAsStr.Length - 1; i < (numAsStr.Length) / 2; i++, j--)
{
if (numAsStr[i] != numAsStr[j])
{
// the corresponding locations do not match
isPalin = false;
// some people forget to break out of the loop
// extremely bad - no need to do any further comparisons
break;
}
}
return isPalin;
}

In the above solution, we cut our execution time by almost half. Better, but not good enough. At this point, the interviewer should have a pretty good idea of your skills on reviewing code and making changes to improve it. Let's assume that he still wants you to keep digging.

public static bool IsPalindrome3(long number)
{
// since the input is a number, one very fast way way would be 
// to reverse the number and compare to original
long revNum = ReverseNumber(number);

return (number == revNum);
}

private static long ReverseNumber(long number)
{
long retVal = 0;
do
{
// get the last digit by using the modulo (remainder) operator
retVal = (retVal * 10) + (number % 10);
// drop the last digit from the number
number = number / 10;
}
while (number != 0);
return retVal;
}

If you run some performance tests with larges number sets on the above solutions, you will find that the 3rd solution is the most performant. I hope this example has shown you one way to attempt to solve a coding question. Remember, it is not just about the answer. The interviewer is trying to find your ability to think, debug and troubleshoot a problem.