LeetCode-in-Java

78. Subsets

Medium

Given an integer array nums of unique elements, return all possible subsets (the power set).

The solution set must not contain duplicate subsets. Return the solution in any order.

Example 1:

Input: nums = [1,2,3]

Output: [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]

Example 2:

Input: nums = [0]

Output: [[],[0]]

Constraints:

• 1 <= nums.length <= 10
• -10 <= nums[i] <= 10
• All the numbers of nums are unique.

To solve the “Subsets” problem in Java with the Solution class, follow these steps:

1. Define a method subsets in the Solution class that takes an integer array nums as input and returns all possible subsets of nums.
2. Initialize an empty list to store the result subsets.
3. Implement a backtracking algorithm to generate all possible subsets:
   • Define a recursive helper function generateSubsets that takes the current subset, the current index in the array, and the array nums as parameters.
   • Base case: If the current index is equal to the length of nums, add the current subset to the result list.
   • Recursive case:
     • Include the current element in the subset and recursively call generateSubsets with the next index.
     • Exclude the current element from the subset and recursively call generateSubsets with the next index.
4. Call the generateSubsets function with an empty subset and the starting index 0.
5. Return the list containing all subsets.

Here’s the implementation of the subsets method in Java:

import java.util.ArrayList;
import java.util.List;

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        generateSubsets(new ArrayList<>(), 0, nums, result);
        return result;
    }
    
    private void generateSubsets(List<Integer> subset, int index, int[] nums, List<List<Integer>> result) {
        // Base case: add the current subset to the result list
        result.add(new ArrayList<>(subset));
        
        // Recursive case
        for (int i = index; i < nums.length; i++) {
            // Include the current element in the subset
            subset.add(nums[i]);
            // Recursively generate subsets starting from the next index
            generateSubsets(subset, i + 1, nums, result);
            // Exclude the current element from the subset
            subset.remove(subset.size() - 1);
        }
    }
}

This implementation uses backtracking to generate all possible subsets of the input array nums. It has a time complexity of O(2^N), where N is the number of elements in the input array.

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