Description

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

Notice

Elements in a subset must be in

• non-descending order.
• The solution set must not contain duplicate subsets.

Example

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

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

Solution

一道recursive BFS的模板题。
result.add(new ArrayList<Integer>(list));copy一个新的数组加进result里
nums = []的corner case不能放到跟null一起返回new ArrayList<List<Integer>>() which is[]而不是[[]]
subsetsHelper的直接返回条件是index > nums.length而不是index >= nums.length

public class Solution {

    /*
     * @param nums: A set of numbers
     * @return: A list of lists
     */
    public List<List<Integer>> subsets(int[] nums) {
        // write your code here
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (nums == null) {
            return result;
        }

        Arrays.sort(nums);
        ArrayList<Integer> list = new ArrayList<Integer>();
        subsetsHelper(nums, result, list, 0);
        return result;
    }

    private void subsetsHelper(int[] nums, List<List<Integer>> result, ArrayList<Integer> list, int index) {
        if (index > nums.length) {
            return;
        }
        result.add(new ArrayList<Integer>(list));
        for (int i = index; i < nums.length; i++) {
            list.add(nums[i]);
            subsetsHelper(nums, result, list, i + 1);
            list.remove(list.size() - 1);
        }
    }
}