diff --git a/src/main/java/com/thealgorithms/maths/PerfectNumber.java b/src/main/java/com/thealgorithms/maths/PerfectNumber.java index cd57000b7..896b8cef8 100644 --- a/src/main/java/com/thealgorithms/maths/PerfectNumber.java +++ b/src/main/java/com/thealgorithms/maths/PerfectNumber.java @@ -6,20 +6,10 @@ package com.thealgorithms.maths; * has divisors 1, 2 and 3 (excluding itself), and 1 + 2 + 3 = 6, so 6 is a * perfect number. * - *
* link:https://en.wikipedia.org/wiki/Perfect_number */ public class PerfectNumber { - public static void main(String[] args) { - assert isPerfectNumber(6); - /* 1 + 2 + 3 == 6 */ - assert !isPerfectNumber(8); - /* 1 + 2 + 4 != 8 */ - assert isPerfectNumber(28); - /* 1 + 2 + 4 + 7 + 14 == 28 */ - } - /** * Check if {@code number} is perfect number or not * @@ -27,6 +17,8 @@ public class PerfectNumber { * @return {@code true} if {@code number} is perfect number, otherwise false */ public static boolean isPerfectNumber(int number) { + if (number <= 0) + return false; int sum = 0; /* sum of its positive divisors */ for (int i = 1; i < number; ++i) { @@ -36,4 +28,38 @@ public class PerfectNumber { } return sum == number; } + + /** + * Check if {@code n} is perfect number or not + * + * @param n the number + * @return {@code true} if {@code number} is perfect number, otherwise false + */ + public static boolean isPerfectNumber2(int n) { + if (n <= 0) + return false; + int sum = 1; + double root = Math.sqrt(n); + + /* + * We can get the factors after the root by dividing number by its factors + * before the root. + * Ex- Factors of 100 are 1, 2, 4, 5, 10, 20, 25, 50 and 100. + * Root of 100 is 10. So factors before 10 are 1, 2, 4 and 5. + * Now by dividing 100 by each factor before 10 we get: + * 100/1 = 100, 100/2 = 50, 100/4 = 25 and 100/5 = 20 + * So we get 100, 50, 25 and 20 which are factors of 100 after 10 + */ + for (int i = 2; i <= root; i++) { + if (n % i == 0) { + sum += i + n / i; + } + } + + // if n is a perfect square then its root was added twice in above loop, so subtracting root from sum + if (root == (int) root) + sum -= root; + + return sum == n; + } } diff --git a/src/test/java/com/thealgorithms/maths/PerfectNumberTest.java b/src/test/java/com/thealgorithms/maths/PerfectNumberTest.java new file mode 100644 index 000000000..92512e99e --- /dev/null +++ b/src/test/java/com/thealgorithms/maths/PerfectNumberTest.java @@ -0,0 +1,21 @@ +package com.thealgorithms.maths; + +import static org.junit.jupiter.api.Assertions.*; +import org.junit.jupiter.api.Test; + +class PerfectNumberTest { + + @Test + public void perfectNumber() { + int trueTestCases[] = { 6, 28, 496, 8128, 33550336 }; + int falseTestCases[] = { -6, 0, 1, 9, 123 }; + for (Integer n : trueTestCases) { + assertTrue(PerfectNumber.isPerfectNumber(n)); + assertTrue(PerfectNumber.isPerfectNumber2(n)); + } + for (Integer n : falseTestCases) { + assertFalse(PerfectNumber.isPerfectNumber(n)); + assertFalse(PerfectNumber.isPerfectNumber2(n)); + } + } +}