Palindroms

Palindroms are strings or numbers that start and end with the same character sequence, for example ABCBA or 12321.

Is Number a Palindrom?

public class IsNumberPalindrom {

    public static void main(String... args) {
        IsNumberPalindrom is = new IsNumberPalindrom();

        System.out.println(is.isPalindrome(1));
        System.out.println(is.isPalindrome(-1));
        System.out.println(is.isPalindrome(1221));
    }

    public boolean isPalindrome(int x) {
        String text = String.valueOf(x);
        int start = 0;
        int end = text.length() - 1;
        while(start < end) {
            if (text.charAt(start) != text.charAt(end)) {
                return false;
            }
            start++;
            end--;
        }
        return true;
    }
}

Lychrel Number

Every number that does not become palindrom after couple of interations is called Lychrel number. Here is an example.

If we send there number like 1960000, it finds 3 palindroms in 1000 interactions. Interesting is that if we use 196 (Lychrel number), we are not able to find Palindrom (nobody found one yet).

boolean isPalindrom(String word) {
  int start = 0;
  int end = word.length() - 1;
  while (start < end) {
    if (word.charAt(start) != word.charAt(end)) {
      return false;
    }
    start++;
    end--;
  }
  return true;
}

BigInteger canidate = 1960000
BigInteger temp = canidate
(1..1000).forEach {
  temp = temp.add(new BigInteger(temp.toString().reverse()))
  if (isPalindrom(temp.toString())) {
    println "Iteration: $it found ${temp}"
  }
}

When we run this for 1960000, the algorithm finds the following palindroms.

Iteration: 1 found 1960691
Iteration: 5 found 27766772
Iteration: 12 found 8434774348