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],
  []
  ]
  

可以推算出如下结论，加入S集合中有n个元素，那么一共有pow(2,n)中子集合。假如S集合有4个元素，这时共需要求出16中子集合，不要忘了还需要对这些子集合进行排序.可以分别用0000-ffff对着16个元素编号，然后通过做一些逻辑运算，可以求出每个集合。C++代码如下，基本是10ms:)：

class Solution {
public:
vector<vector<int> > subsets(vector<int> &S) {
    vector<vector<int> > ret;
    vector<int> subset;

    for (int i = 0; i < (1 << S.size()); i++)
    {
        subset.clear();
        for (int j = 0; j < S.size(); j++)
        {
            if ((1 << j) & i)
                subset.push_back(S[j]);
        }
        sort(subset.begin(), subset.end());
        ret.push_back(subset);
    }

    return ret;
}
};