Add HappyNumber task solutions

Author: andrei-punko

src/main/java/by/andd3dfx/numeric/HappyNumber.java

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+package by.andd3dfx.numeric;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * <pre>
+ * <a href="https://leetcode.com/problems/happy-number/description/">Task description</a>
+ *
+ * Write an algorithm to determine if a number n is happy.
+ * A happy number is a number defined by the following process:
+ *     Starting with any positive integer, replace the number by the sum of the squares of its digits.
+ *     Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
+ *     Those numbers for which this process ends in 1 are happy.
+ * Return true if n is a happy number, and false if not.
+ *
+ * Example 1:
+ *   Input: n = 19
+ *   Output: true
+ *   Explanation:
+ *   12 + 92 = 82
+ *   82 + 22 = 68
+ *   62 + 82 = 100
+ *   12 + 02 + 02 = 1
+ *
+ * Example 2:
+ *   Input: n = 2
+ *   Output: false
+ * </pre>
+ */
+public class HappyNumber {
+
+    public static boolean isHappyUsingMemory(int n) {
+        Set<Integer> set = new HashSet<>();
+
+        var tmp = getSumOfSquares(n);
+        while (tmp != 1 && set.add(tmp)) {
+            tmp = getSumOfSquares(tmp);
+        }
+        return tmp == 1;
+    }
+
+    public static boolean isHappyUsing2Pointers(int n) {
+        var p1 = getSumOfSquares(n);
+        var p2 = getSumOfSquares(p1);
+
+        do {
+            // p1: 1 step
+            p1 = getSumOfSquares(p1);
+
+            // p2: 2 steps
+            p2 = getSumOfSquares(p2);
+            if (p1 == p2) {
+                return p1 == 1;
+            }
+            p2 = getSumOfSquares(p2);
+            if (p1 == p2) {
+                return p1 == 1;
+            }
+        } while (true);
+    }
+
+    private static int getSumOfSquares(int n) {
+        var result = 0;
+        while (n > 0) {
+            var remainder = n % 10;
+            result += remainder * remainder;
+            n /= 10;
+        }
+        return result;
+    }
+}

src/test/java/by/andd3dfx/numeric/HappyNumberTest.java

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+package by.andd3dfx.numeric;
+
+import org.junit.Test;
+
+import static by.andd3dfx.numeric.HappyNumber.isHappyUsing2Pointers;
+import static by.andd3dfx.numeric.HappyNumber.isHappyUsingMemory;
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class HappyNumberTest {
+
+    @Test
+    public void testIsHappyUsingMemory() {
+        assertThat(isHappyUsingMemory(1)).isTrue();
+        assertThat(isHappyUsingMemory(2)).isFalse();
+        assertThat(isHappyUsingMemory(19)).isTrue();
+        assertThat(isHappyUsingMemory(25)).isFalse();
+    }
+
+    @Test
+    public void testIsHappyUsing2Pointers() {
+        assertThat(isHappyUsing2Pointers(1)).isTrue();
+        assertThat(isHappyUsing2Pointers(2)).isFalse();
+        assertThat(isHappyUsing2Pointers(19)).isTrue();
+        assertThat(isHappyUsing2Pointers(25)).isFalse();
+    }
+}