[leedcode 78] Subsets

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

Note:

• Elements in a subset must be in non-descending order.
• 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],
  []
]

public class Solution {
    //DFS
    //注意：本题是要所有的组合，所以跟全排列的不同是，深搜的判断条件是只要符合要求的就加入最终结果
    //注意递归函数中，getSub的参数意义:从start开始
    //注意需要排序，题目暗含递增排序
    //时间复杂度O(2^n),空间复杂度O(n)
    List<List<Integer>> res;
    List<Integer> seq;
    public List<List<Integer>> subsets(int[] nums) {
        res=new ArrayList<List<Integer>>();
        seq=new ArrayList<Integer>();
        Arrays.sort(nums);/////////
        getSub(nums,0,nums.length);
        return res;
    }
    public void getSub(int[] nums,int start,int end){
        if(start<=end){///////
            res.add(new ArrayList<Integer>(seq));
            
        }
        for(int i=start;i<end;i++){
            seq.add(nums[i]);
            getSub(nums,i+1,end);//////
            seq.remove(seq.size()-1);
        }
    }
}