[Leetcode] Subsets

题目：

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],  []]

思路：递归列举所有情况，每一个node有两种选择，加入到集合中或不加入到集合中。

class Solution {public:    vector<int> trace;        void subsets_helper(const vector<int>& s, int pos, vector<vector<int>>& result) {        if (pos == (int)s.size()) {            result.push_back(trace);            return;        }        // two possibilities: to put the element in or not        subsets_helper(s, pos + 1, result);        trace.push_back(s[pos]);        subsets_helper(s, pos + 1, result);        trace.pop_back();    }        vector<vector<int> > subsets(vector<int> &S) {        vector<vector<int>> result;        sort(S.begin(), S.end());        subsets_helper(S, 0, result);        return result;    }};

总结：每一个node两个分支，复杂度O(2^n).