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],  []]

这个题目和上一个combinations题目一样的，这个题目只是在进行下一个level之前把当前的level的结果push上去而已。比较tricky的有这么几个地方：

1. 需要sort一下来确保是non-descending

2. 不管怎样都要push一个空的上去。

其他的和combinations 那个题目一样。

class Solution {public:    vector<vector<int> > subsets(vector<int> &S) {        vector< vector<int> > res;        vector<int> solution;        res.push_back(solution); // non data        if (S.size()==0)        return res;        sort(S.begin(),S.end()); // make sure it is ascending but not descending        helper(res,solution, S, 0); //backtracking        return res;    }    void helper(vector< vector<int> >& res, vector<int>& solution, vector<int>& input, int level){    for (int i=level; i<input.size();i++){    solution.push_back(input[i]);    res.push_back(solution);    helper(res,solution,input,i+1);    solution.pop_back();    }    }};