【leetcode刷题笔记】Subsets

Given a set of distinct integers, S, 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 S = [1,2,3], a solution is:

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

---

题解：实现一个递归的函数生成长度为level的所有子集放入结果中，然后循环的置这个level为0~S.length，即可生成所有的子集。

代码如下：

public class Solution {
    public void subsetsDfs(int[] s,int level,int pos,ArrayList<Integer> result,List<List<Integer>> answer){
        if(result.size() == level){
            answer.add(new ArrayList<Integer>(result));
            return;
        }
        for(int i = pos;i < s.length;i++){
            result.add(s[i]);
            subsetsDfs(s, level, i+1, result, answer);
            result.remove(result.size()-1);
        }
    }
    public List<List<Integer>> subsets(int[] S) {
        Arrays.sort(S);
        List<List<Integer>> answer = new ArrayList<List<Integer>>();
        ArrayList<Integer> result = new ArrayList<Integer>();
        
        for(int i = 0;i <= S.length;i++)
            subsetsDfs(S, i, 0, result, answer);
        return answer;
    }
}