78. Subsets

Given a set of distinct integers, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example,

If nums = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

解题思路为recursion。

remember start position and do backtracking.

public class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        helper(nums, 0, new ArrayList<Integer>(), res);
        return res;
    }
    
    private void helper(int[] nums, int start, List<Integer> subset, List<List<Integer>> res) {
        res.add(new ArrayList<Integer>(subset));
        if(start == nums.length) return;
        for(int i = start; i < nums.length; i++) {
            subset.add(nums[i]);
            helper(nums, i+1, subset, res);
            subset.remove(subset.size()-1);
        }
    }
}