Java: Find the number of seed Lychrel number candidates and related numbers for n in the range 1..10000 inclusive.

Last update on March 11 2025 11:10:13 (UTC/GMT +8 hours)

Seed Lychrel Numbers

Write a Java program to find the number of seed Lychrel number candidates and related numbers for n in the range 1..10000 inclusive. (With a 500-iteration limit).

A Lychrel number is a natural number that cannot form a palindrome through the iterative process of repeatedly reversing its digits and adding the resulting numbers. This process is sometimes called the 196-algorithm, after the most famous number associated with the process.
The first few Lychrel numbers are 196, 295, 394, 493, 592, 689, 691, 788, 790, 879, 887, ... .

Pictorial Presentation:

Sample Solution:

Java Code:

// https://rosettacode.org/
import java.math.BigInteger;
import java.util.*;
 public class Example5 {
     static Map<BigInteger, Tuple> cache = new HashMap<>();
     static class Tuple {
        final Boolean flag;
        final BigInteger bi;
 
        Tuple(boolean f, BigInteger b) {
            flag = f;
            bi = b;
        }
    }
     static BigInteger rev(BigInteger bi) {
        String s = new StringBuilder(bi.toString()).reverse().toString();
        return new BigInteger(s);
    }
     static Tuple lychrel(BigInteger n) {
        Tuple res;
        if ((res = cache.get(n)) != null)
            return res;
 
        BigInteger r = rev(n);
        res = new Tuple(true, n);
        List<BigInteger> seen = new ArrayList<>();
         for (int i = 0; i < 500; i++) {
            n = n.add(r);
            r = rev(n);
 
            if (n.equals(r)) {
                res = new Tuple(false, BigInteger.ZERO);
                break;
            }
             if (cache.containsKey(n)) {
                res = cache.get(n);
                break;
            }
             seen.add(n);
        }
         for (BigInteger bi : seen)
            cache.put(bi, res);
         return res;
    }
     public static void main(String[] args) {
         List<BigInteger> seeds = new ArrayList<>();
        List<BigInteger> related = new ArrayList<>();
        List<BigInteger> palin = new ArrayList<>();
 
        for (int i = 1; i <= 10_000; i++) {
            BigInteger n = BigInteger.valueOf(i);
             Tuple t = lychrel(n);
             if (!t.flag)
                continue;
             if (n.equals(t.bi))
                seeds.add(t.bi);
            else
                related.add(t.bi);
 
            if (n.equals(t.bi))
                palin.add(t.bi);
        }
         System.out.printf("%d Lychrel seeds: %s%n", seeds.size(), seeds);
        System.out.printf("%d Lychrel related%n", related.size());
        System.out.printf("%d Lychrel palindromes: %s%n", palin.size(), palin);
    }
}

Sample Output:

5 Lychrel seeds: [196, 879, 1997, 7059, 9999]                                                                 
244 Lychrel related                                                                                           
5 Lychrel palindromes: [196, 879, 1997, 7059, 9999]

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Java program to identify candidate Lychrel numbers in a range using the reverse-and-add process with a user-defined iteration limit.
• Write a Java program to display the sequence of reverse-and-add operations for a suspected Lychrel number.
• Write a Java program to count Lychrel seeds and related numbers using multi-threading for performance improvement.
• Write a Java program to analyze and cache intermediate results in the Lychrel detection process to avoid redundant calculations.

Java Code Editor:

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