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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],  []]

子集问题我们用递归来做还是比较简单的，具体过程见图示：

//CODE

class Solution {public:	void subRe(vector
   
    
      >& re, vector
     
       &s,int j)	{		if(s.size() <= j)			return;		int size_ = re.size();		for(int i = 0; i < size_; ++i)		{			vector
      
        copy(re[i]);			copy.push_back(s[j]);			re.push_back(copy);		}		subRe(re, s, j + 1);	}	vector
       
        
          > subsets(vector
         
           &S) { vector
          
           
             > re; sort(S.begin(),S.end()); vector
            
              sol; re.push_back(sol); subRe(re, S, 0); return re; }};