let 78 Subsets

Given a set of distinct integers, nums, return all possible subsets (the power set).

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],
  []
]

主题思想： 这个和77 题同样是通过回溯完成的，形成了一个比较典型的方法吧

AC 代码：

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> ans=new ArrayList<List<Integer>>();
        Arrays.sort(nums);
        dfs(nums,ans,new ArrayList<Integer>(),0);
        return ans;
    }
    public void dfs(int[] nums,List<List<Integer>>ans,List<Integer> tmp,int start){

        ans.add(new ArrayList<Integer>(tmp));
        for(int i=start;i<nums.length;i++){
            tmp.add(nums[i]);
            dfs(nums,ans,tmp,i+1);
            tmp.remove(tmp.size()-1);
        }
    }
}