Subsets - https://leetcode.com/problems/subsets/description/

gistfile1.txt

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
      // check
      if (nums == null) {
        throw new IllegalArgumentException();
      }
      
      List<List<Integer>> results = new LinkedList<List<Integer>>();
      
      // prepare to recursive
      List<Integer> prefix = new LinkedList<Integer>();
      
      // lets recursive to process all
      recursiveSubset(nums, 0, prefix, results);
      
      return results;
      
    }
  
    private void recursiveSubset(int[] nums, int index, List<Integer> prefix, List<List<Integer>> results) {
      
      // stopping criteria
      if (index > nums.length) {
        return;
      }
      
      // Process the prefix b4 we modify
      results.add(new LinkedList<Integer>(prefix));
      
      for (int i = index; i < nums.length; i++) {
        int curNum = nums[i];
        
        prefix.add(curNum);
        recursiveSubset(nums, i + 1, prefix, results);
        
        // remove last prefix;
        prefix.remove(prefix.size() - 1);
      }
    }
}

https://leetcode.com/problems/subsets/discuss/27281/A-general-approach-to-backtracking-questions-in-Java-(Subsets-Permutations-Combination-Sum-Palindrome-Partitioning)