Generate All Subsets Powerset in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add the current subset to the result list.

DSA Java

result.add(new ArrayList<>(subset[1]));

Drag options to blanks, or click blank then click option'

A[]

B{}

C)

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets instead of parentheses.

Trying to add the subset directly without copying.

We use parentheses to call the ArrayList constructor to create a copy of the current subset.

2fill in blank

medium

Complete the code to add the current element nums[index] to the subset.

DSA Java

subset.[1](nums[index]);

Drag options to blanks, or click blank then click option'

Aadd

Bappend

Cinsert

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using append which is not a method in ArrayList.

Using push which is from other languages.

In Java's ArrayList, the method to add an element is add().

3fill in blank

hard

Fix the error in the recursive call to generate subsets.

DSA Java

generateSubsets(nums, [1] + 1, subset, result);

Drag options to blanks, or click blank then click option'

Aresult.size()

Bi

Csubset.size()

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using i which is undefined in this scope.

Using subset.size() or result.size() which are unrelated.

The recursive call should move to the next index, so we add 1 to the current index variable.

4fill in blank

hard

Fill both blanks to remove the last element from subset after recursion.

DSA Java

subset.[1](subset.[2]() - 1);

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Aremove

Bsize

Clength

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which is not a method in ArrayList.

Using length() which is not a method in ArrayList.

We remove the element at the last index which is size() - 1 in ArrayList.

5fill in blank

hard

Fill all three blanks to complete the method signature and base case check.

DSA Java

public void generateSubsets(int[] [1], int [2], List<Integer> subset, List<List<Integer>> [3]) {
    if ([2] == [1].length) {
        [3].add(new ArrayList<>(subset));
        return;
    }
    // rest of code
}

Drag options to blanks, or click blank then click option'

Anums

Bindex

Cresult

Dsubset

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing parameter names or using undefined variables.

Not matching parameter names with those used inside the method.

The method takes nums array, index integer, subset list, and result list as parameters. The base case checks if index equals nums.length and adds subset to result.

Practice

1. What does the term powerset mean in the context of subsets?

easy

A. All possible subsets of a given set, including empty and full set

B. Only the subsets with exactly two elements

C. The set of elements that are not in the original set

D. The largest subset of a given set

Solution

1. Step 1: Understand the definition of powerset

   The powerset of a set is the collection of all possible subsets, including the empty set and the set itself.

2. Step 2: Compare options with definition

   All possible subsets of a given set, including empty and full set correctly describes the powerset as all subsets. Other options describe partial or unrelated concepts.

3. Final Answer:

   All possible subsets of a given set, including empty and full set -> Option A

4. Quick Check:

   powerset = all subsets [OK]

Hint: Powerset means every subset possible, including empty [OK]

Common Mistakes:

• Thinking powerset excludes empty set
• Confusing powerset with subsets of fixed size
• Assuming powerset means complement sets

2. Which of the following Java method signatures correctly represents a function that returns all subsets of an integer array?

easy

A. public int generateSubsets(int[] nums)

B. public void generateSubsets(int[] nums)

C. public List generateSubsets(int[] nums)

D. public List<Integer> generateSubsets(int nums)

Solution

1. Step 1: Identify return type for all subsets

   All subsets of an integer array require returning a list of lists of integers, so return type should be List.

2. Step 2: Check method parameters and return type

   public List generateSubsets(int[] nums) matches correct return type and parameter. Options A and C have wrong return types, and D has wrong parameter type.

3. Final Answer:

   public List<List<Integer>> generateSubsets(int[] nums) -> Option C

4. Quick Check:

   Return type must hold all subsets = List of List [OK]

Hint: Return type for subsets is List of List of Integers [OK]

Common Mistakes:

• Using int return type instead of list
• Using void return type without output
• Passing single int instead of int array

3. What is the output of the following Java code snippet?

public static List subsets(int[] nums) {
    List result = new ArrayList<>();
    result.add(new ArrayList<>());
    for (int num : nums) {
        int size = result.size();
        for (int i = 0; i < size; i++) {
            List<Integer> subset = new ArrayList<>(result.get(i));
            subset.add(num);
            result.add(subset);
        }
    }
    return result;
}

// Called with nums = {1, 2}

medium

A. [[], [1], [2], [1, 2]]

B. [[1], [2], [1, 2]]

C. [[], [1, 2]]

D. [[1], [2]]

Solution

1. Step 1: Initialize result with empty subset

   Initially, result = [[]].

2. Step 2: Iterate over nums and add new subsets

   For num=1, add [1] to result: result = [[], [1]]. For num=2, add [2] and [1,2]: result = [[], [1], [2], [1, 2]].

3. Final Answer:

   [[], [1], [2], [1, 2]] -> Option A

4. Quick Check:

   All subsets of {1,2} = [[], [1], [2], [1, 2]] [OK]

Hint: Start with empty set, add each number to existing subsets [OK]

Common Mistakes:

• Forgetting to include empty subset
• Adding subsets in wrong order
• Missing combined subsets like [1, 2]

4. Identify the error in the following Java code that attempts to generate all subsets of an integer array:

public static List subsets(int[] nums) {
    List result = new ArrayList<>();
    for (int num : nums) {
        int size = result.size();
        for (int i = 0; i < size; i++) {
            List<Integer> subset = new ArrayList<>(result.get(i));
            subset.add(num);
            result.add(subset);
        }
    }
    result.add(new ArrayList<>());
    return result;
}

medium

A. The input array should be converted to a list first

B. The inner loop should run from size-1 to 0

C. The method should return an int, not a list

D. Empty subset is added at the end, should be added before loop

Solution

1. Step 1: Check when empty subset is added

   Empty subset must be added before starting to build subsets, so that initial subsets exist.

2. Step 2: Analyze code flow

   Here, empty subset is added after loops, so result.size() is zero initially and inner loop never runs, producing empty result except empty subset at end.

3. Final Answer:

   Empty subset is added at the end, should be added before loop -> Option D

4. Quick Check:

   Empty subset must be first for subsets to build [OK]

Hint: Add empty subset before loops to start building subsets [OK]

Common Mistakes:

• Adding empty subset after loops
• Incorrect loop ranges causing no subsets
• Returning wrong data type

5. Given an integer array nums that may contain duplicates, which modification to the standard subsets generation algorithm ensures the powerset contains no duplicate subsets?

hard

A. Only generate subsets of fixed size to avoid duplicates

B. Sort the array first and skip adding subsets that duplicate the last added subset

C. Generate subsets normally without changes; duplicates are automatically handled

D. Use a HashSet to store subsets and convert back to list at the end

Solution

1. Step 1: Understand duplicates in subsets

   Duplicates in input cause duplicate subsets if not handled.

2. Step 2: Use sorting and skip duplicates

   Sorting groups duplicates together. While generating subsets, skip adding subsets that duplicate the last added subset to avoid duplicates.

3. Final Answer:

   Sort the array first and skip adding subsets that duplicate the last added subset -> Option B

4. Quick Check:

   Sort + skip duplicates = unique subsets [OK]

Hint: Sort input and skip duplicates during subset generation [OK]

Common Mistakes:

• Ignoring duplicates causing repeated subsets
• Using HashSet but forgetting order or conversion
• Assuming duplicates don't affect subsets