78. Subsets

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.

Solution

class Solution {
    
     public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> subsets = new ArrayList<>();
        int n = nums.length;
        for (int i = (int) Math.pow(2, n); i < (int) Math.pow(2, n + 1); i++) {
            // generate bitmasks from 00.000 to 111..111
            String bitmask = Integer.toBinaryString(i).substring(1);
            List<Integer> curr = new ArrayList<>();
            for (int j = 0; j < n; j++) {
                if (bitmask.charAt(j) == '1') curr.add(nums[j]);
            }
            subsets.add(curr);
        }
        return subsets;
    }
    
     public List<List<Integer>> subsets3(int[] nums) {
        List<List<Integer>> subsets = new ArrayList<>();
        List<Integer> current = new ArrayList<>();
        dfs(0, nums, current, subsets);
        return subsets;
    }
    
    void dfs(int index, int[] nums, List<Integer> current, List<List<Integer>> subsets) {
        if (index == nums.length) {
             subsets.add(new ArrayList<>(current));
            return;
        }
        
        current.add(nums[index]);
        dfs(index + 1, nums, current, subsets);
        
        current.remove(current.size() - 1);
        dfs(index + 1, nums, current, subsets);
    }
    
    public List<List<Integer>> subsets2(int[] nums) {
        List<List<Integer>> subsets = new ArrayList<>();
        List<Integer> current = new ArrayList<>();
        generateSubsets(0, nums, current, subsets);
        return subsets;
    }
    
    void generateSubsets(int index, int[] nums, List<Integer> current, List<List<Integer>> subsets) {
        
        subsets.add(new ArrayList<>(current));
        
        for (int i = index; i < nums.length; i++) {
            current.add(nums[i]);
            generateSubsets(i + 1, nums, current, subsets);
            current.remove(current.size() - 1);
        }
    }
}

Solution 2021-11-22

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> curr = new ArrayList<>();
        dfs(0, nums, res, curr);
        return res;
    }
    
    void dfs(int index, int[] nums, List<List<Integer>> res, List<Integer> curr) {
        if (index == nums.length) {
            res.add(List.copyOf(curr));
            return;
        }
        
        curr.add(nums[index]);
        
        dfs(index + 1, nums, res, curr);
        
        curr.remove(curr.size() - 1);
        dfs(index + 1, nums, res, curr);
    }
}