feat: recursion subsets added

src/main/java/com/thealgorithms/Recursion/Subsets.java

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+package com.thealgorithms.Recursion;
+
+// program to find power set of a string
+
+import java.util.ArrayList;
+
+public class Subsets {
+    public static void main(String[] args) {
+        String str = "abc";
+        ArrayList<String> res = new ArrayList<>();
+        subset("",str,res);
+        System.out.println(res);
+    }
+
+    public static void subset(String p,String up,ArrayList<String> res){
+        if(up.isEmpty()){
+            res.add(p);
+            return;
+        }
+
+        // Taking the character
+        char ch = up.charAt(0);
+        // Adding the character in the recursion
+        subset(p+ch,up.substring(1),res);
+        // Not adding the character in the recursion
+        subset(p,up.substring(1),res);
+    }
+}

src/test/java/com/thealgorithms/Recursion/SubsetsTest.java

@@ -0,0 +1,28 @@
+package com.thealgorithms.Recursion;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+
+public class SubsetsTest {
+
+    @Test
+    void subset(){
+        String str = "abc";
+        ArrayList<String> ans = new ArrayList<>();
+        ArrayList<String> actual = new ArrayList<>();
+        ans.add("abc");
+        ans.add("ab");
+        ans.add("ac");
+        ans.add("a");
+        ans.add("bc");
+        ans.add("b");
+        ans.add("c");
+        ans.add("");
+
+        Subsets.subset("",str,actual);
+        assertEquals(ans.toString(),actual.toString());
+    }
+}