两种方法：

一. 回溯：

C++:

class Solution {
public:
    vector<vector<int>&gt;result;
    vector<int>path;
    void backtracing(vector<int>S,int len,int index){
        result.push_back(path);
        for(int i=index;i<len;++i){ path.push_back(s[i]); backtracing(s,len,index+1); path.pop_back(); } vector<vector<int> &gt; subsets(vector<int> &amp;S) {
        result.clear();
        path.clear();
        sort(S.begin(),S.end());
        backtracing(S,S.size(),0);
        return result;
    }
};

JAVA:

import java.util.*;

public class Solution {
    ArrayList<arraylist<integer>&gt;result=new ArrayList&lt;&gt;();
    ArrayList<integer>path=new ArrayList<integer>();
    void backtracing(int[] S,int len,int index){
        result.add(new ArrayList&lt;&gt;(path));//不能跟C++一样直接result.add(path).......
        for(int i=index;i<len;++i){ path.add(s[i]); backtracing(s,len,i+1); path.remove(path.size()-1); } public arraylist<arraylist<integer>&gt; subsets(int[] S) {
        Arrays.sort(S);
        backtracing(S,S.length,0);
        return result;
    }
}

python3:

#
# 
# @param A int整型一维数组 
# @return int整型二维数组
#
class Solution:
    def backtracing(self,A,length:int,index:int,ans1,path1):
        ans1.append(path1.copy())
        for i in range(index,length):
            path1.append(A[i])
            self.backtracing(A, length,i+1,ans1,path1)
            path1.pop()
    def subsets(self , A ):
        # write code here
        length=len(A)
        A.sort()
        ans=[]
        path=[]
        self.backtracing(A, length, 0,ans,path)
        return ans

二.迭代：

C++:

class Solution {
public:
    vector<vector<int> > subsets(vector<int> &S) {
        vector<vector<int>>ans;
        int n=S.size();
        //注意要先push一个子集
        ans.push_back({});
        for(int i=0;i<n;++i){
            int size=ans.size();
            for(int j=0;j<size;++j){
                vector<int>path(ans[j]);
                path.push_back(S[i]);
                ans.push_back(path);
            }
        }
        return ans;
    }
};

JAVA:

import java.util.*;

public class Solution {
    public ArrayList<ArrayList<Integer>> subsets(int[] S) {
        int n=S.length;
        ArrayList<ArrayList<Integer>>ans=new ArrayList<>();
        ans.add(new ArrayList<Integer>());
        for(int i=0;i<n;++i){
            int size=ans.size();
            for(int j=0;j<size;++j){
                ArrayList<Integer>path=new ArrayList<Integer>();
                path.addAll(ans.get(j));
                path.add(S[i]);
                ans.add(path);
            }
        }
        return ans;
    }
}

python:

#
# 
# @param A int整型一维数组 
# @return int整型二维数组
#
class Solution:
    def subsets(self , A ):
        # write code here
        res=[[]]
        for item in A:
            res+=[i+[item] for i in res]
        return res