Problem Description

Write a Java program to check if a number is a Keith number. A n-digit number N is called a Keith number if it appears in a sequence generated using its digits. The sequence starts with the digits of N, and each subsequent term is the sum of the previous n terms.

Code

KeithNumber.java

import java.util.*;

public class KeithNumber {
    public static void main(String[] args) {
        int num = 197;
        ArrayList<Integer> terms = new ArrayList<>();
        int temp = num;
        int n = 0; // number of digits

        while (temp > 0) {
            terms.add(0, temp % 10);
            temp /= 10;
            n++;
        }

        int nextTerm = 0;
        while (nextTerm < num) {
            nextTerm = 0;
            for (int i = terms.size() - n; i < terms.size(); i++) {
                nextTerm += terms.get(i);
            }
            terms.add(nextTerm);
            if (nextTerm == num) {
                System.out.println(num + " is a Keith Number.");
                return;
            }
        }
        System.out.println(num + " is not a Keith Number.");
    }
}

Output

197 is a Keith Number.

Explanation

Sequence: 1, 9, 7.
Next Term: 1+9+7 = 17. Sequence: 1, 9, 7, 17.
Next Term: 9+7+17 = 33. Sequence: 1, 9, 7, 17, 33.
...: Eventually reaches 197.

Java Program to Check Keith Number

Problem Description

Code

Output

Explanation

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