Improve comments in SumOfSubset.java

src/main/java/com/thealgorithms/dynamicprogramming/SumOfSubset.java

@@ -1,18 +1,52 @@
 package com.thealgorithms.dynamicprogramming;
 
+/**
+ * A utility class that contains the Sum of Subset problem solution using
+ * recursion.
+ *
+ * The Sum of Subset problem determines whether a subset of elements from a
+ * given array sums up to a specific target value.
+ *
+ * Wikipedia: https://en.wikipedia.org/wiki/Subset_sum_problem
+ */
 public final class SumOfSubset {
+
     private SumOfSubset() {
     }
 
+    /**
+     * Determines if there exists a subset of elements in the array `arr` that
+     * adds up to the given `key` value.
+     *
+     * @param arr The array of integers.
+     * @param num The index of the current element being considered.
+     * @param key The target sum we are trying to achieve.
+     * @return true if a subset of `arr` adds up to `key`, false otherwise.
+     *
+     *         This is a recursive solution that checks for two possibilities at
+     *         each step:
+     *         1. Include the current element in the subset and check if the
+     *         remaining
+     *         elements can sum up to the remaining target.
+     *         2. Exclude the current element and check if the remaining elements
+     *         can
+     *         sum up to the target without this element.
+     */
     public static boolean subsetSum(int[] arr, int num, int key) {
+        // Base case
         if (key == 0) {
-            return true;
+            return true; // subset sum found
         }
         if (num < 0 || key < 0) {
-            return false;
+            return false; // no more elements to consider or key is negative
         }
 
+        // Recursive case
+
+        // Pick the current element and include it in the subset
         boolean include = subsetSum(arr, num - 1, key - arr[num]);
+
+        // Don't pick the current element and exclude it from the subset
         boolean exclude = subsetSum(arr, num - 1, key);
 
         return include || exclude;