problem 20, 21

realDuYuanChao · realDuYuanChao · commit 2436ec7c3e13 · 2020-10-07T14:47:44.000+08:00
diff --git a/src/main/java/com/examplehub/maths/SumOfDigits.java b/src/main/java/com/examplehub/maths/SumOfDigits.java
@@ -12,7 +12,7 @@ public static int sumOfDigits(long number) {
         number = Math.abs(number);
         int sum = 0;
         while (number != 0) {
-            sum += number % 10;
+            sum = (int) (sum + number % 10);
             number /= 10;
         }
         return sum;
diff --git a/src/main/java/com/examplehub/maths/SumOfDivisor.java b/src/main/java/com/examplehub/maths/SumOfDivisor.java
@@ -0,0 +1,36 @@
+package com.examplehub.maths;
+
+public class SumOfDivisor {
+
+    /**
+     * Calculate sum of divisors of a number, including itself.
+     *
+     * @param number the number to be calculated
+     * @return sum of divisors.
+     */
+    public static int sumOfDivisorInclude(int number) {
+        int sum = 0;
+        for (int i = 1; i <= number; i++) {
+            if (number % i == 0) {
+                sum += i;
+            }
+        }
+        return sum;
+    }
+
+    /**
+     * Calculate sum of divisors of a number, excluding itself.
+     *
+     * @param number the number to be calculated
+     * @return sum of divisors.
+     */
+    public static int sumOfDivisorExclude(int number) {
+        int sum = 0;
+        for (int i = 1; i < number; i++) {
+            if (number % i == 0) {
+                sum += i;
+            }
+        }
+        return sum;
+    }
+}
diff --git a/src/main/java/com/examplehub/projecteuler/Problem20.java b/src/main/java/com/examplehub/projecteuler/Problem20.java
@@ -3,6 +3,9 @@
 import com.examplehub.maths.Factorial;
 import com.examplehub.maths.SumOfDigits;
 
+/**
+ * https://projecteuler.net/problem=20
+ */
 public class Problem20 {
     public static int solution1(int number) {
         return SumOfDigits.sumOfDigits(Factorial.factorial(number));
diff --git a/src/main/java/com/examplehub/projecteuler/Problem21.java b/src/main/java/com/examplehub/projecteuler/Problem21.java
@@ -0,0 +1,19 @@
+package com.examplehub.projecteuler;
+
+import com.examplehub.maths.SumOfDivisor;
+
+/**
+ * https://projecteuler.net/problem=21
+ */
+public class Problem21 {
+    public static int solution1(int n) {
+        int sum = 0;
+        for (int i = 1; i < n; i++) {
+            int sumOfDivisor = SumOfDivisor.sumOfDivisorExclude(i);
+            if (SumOfDivisor.sumOfDivisorExclude(sumOfDivisor) == i && sumOfDivisor != i) {
+                sum += i;
+            }
+        }
+        return sum;
+    }
+}
diff --git a/src/test/java/com/examplehub/maths/SumOfDivisorTest.java b/src/test/java/com/examplehub/maths/SumOfDivisorTest.java
@@ -0,0 +1,23 @@
+package com.examplehub.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SumOfDivisorTest {
+    @Test
+    void testSumOfDivisorInclude() {
+        assertEquals(0, SumOfDivisor.sumOfDivisorInclude(0));
+        assertEquals(1, SumOfDivisor.sumOfDivisorInclude(1));
+        assertEquals(3, SumOfDivisor.sumOfDivisorInclude(2));
+        assertEquals(18, SumOfDivisor.sumOfDivisorInclude(10));
+    }
+
+    @Test
+    void testSumOfDivisorExclude() {
+        assertEquals(0, SumOfDivisor.sumOfDivisorExclude(0));
+        assertEquals(0, SumOfDivisor.sumOfDivisorExclude(1));
+        assertEquals(1, SumOfDivisor.sumOfDivisorExclude(2));
+        assertEquals(8, SumOfDivisor.sumOfDivisorExclude(10));
+    }
+}
diff --git a/src/test/java/com/examplehub/projecteuler/Problem21Test.java b/src/test/java/com/examplehub/projecteuler/Problem21Test.java
@@ -0,0 +1,14 @@
+package com.examplehub.projecteuler;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class Problem21Test {
+
+    @Test
+    void test() {
+        assertEquals(504, Problem21.solution1(1000));
+        assertEquals(31626, Problem21.solution1(10000));
+    }
+}

Original file line number	Diff line number	Diff line change
`@@ -12,7 +12,7 @@ public static int sumOfDigits(long number) {`
`12`	`12`	`number = Math.abs(number);`
`13`	`13`	`int sum = 0;`
`14`	`14`	`while (number != 0) {`
`15`		`- sum += number % 10;`
	`15`	`+ sum = (int) (sum + number % 10);`
`16`	`16`	`number /= 10;`
`17`	`17`	`}`
`18`	`18`	`return sum;`