17. 子集

给定一个含不同整数的集合，返回其所有的子集

样例

如果 S = [1,2,3]，有如下的解：

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

public class Solution {
    
    /*
     * @param nums: A set of numbers
     * @return: A list of lists
     */
    public List> subsets(int[] nums) {
        // write your code here
        List> result = new ArrayList<>();
		if (nums == null) {
			return result;
		}
		Arrays.sort(nums);
		List item = new ArrayList<>();
		for (int i = 0; i <= nums.length; i++) {
			item.clear();
			dfs(nums, 0, i, item, result);
		}
		return result;
	} 
	
	private static void dfs(int[] nums, int start, int size, List item, List> result) {
		if (item.size() == size) {
			result.add(new ArrayList<>(item));
			return;
		}
		for (int i = start; i < nums.length; i++) {
			item.add(nums[i]);
			dfs(nums, i+1, size, item, result);
			item.remove(item.size() - 1);
		}
	}
    
}