Added a harmonic number calculation algorithm

Author: Farai Mugaviri

src/main/java/com/thealgorithms/maths/HarmonicNumber.java

@@ -0,0 +1,36 @@
+package com.thealgorithms.maths;
+
+/**
+ * A class for calculating harmonic numbers.
+ * The harmonic series is the divergent infinite series: 1 + 1/2 + 1/3 + 1/4 + ...
+ */
+public final class HarmonicNumber {
+    
+    private HarmonicNumber() {}
+    
+    /**
+     * Calculates the nth harmonic number.
+     * The harmonic series is the sum: 1 + 1/2 + 1/3 + ... + 1/n
+     * This series appears in various mathematical and practical applications.
+     *
+     * For example:
+     * H(1) = 1
+     * H(2) = 1 + 1/2 = 1.5
+     * H(3) = 1 + 1/2 + 1/3 ≈ 1.833
+     * H(4) = 1 + 1/2 + 1/3 + 1/4 ≈ 2.083
+     *
+     * @param numTerms the number of terms to sum (n)
+     * @return the nth harmonic number
+     * @throws IllegalArgumentException if numTerms is less than 1
+     */
+    public static double of(int numTerms) {
+        if (numTerms < 1) {
+            throw new IllegalArgumentException("Number of terms must be positive");
+        }
+        double harmonicSum = 0.0;
+        for (int termIndex = 1; termIndex <= numTerms; termIndex++) {
+            harmonicSum += 1.0 / termIndex;
+        }
+        return harmonicSum;
+    }
+}