leetcode5:subsets问题

问题描述：

   Given a set of distinct integers, nums, 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 nums = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

方法一：迭代

class Solution {
public:
   vector<vector<int>> subsets(vector<int>& nums) {
      sort(nums.begin(),nums.end());
       vector<int> v;
       vector<vector<int>> res;
       res.push_back(v);
       for(int i=0;i<nums.size();i++)
       {
           for(int j=res.size()-1;j>=0;j--)
           {
               v.assign(res[j].begin(),res[j].end());
               v.push_back(nums[i]);
               res.push_back(v);
           }
       }
       return res;
   }
}; 

方法二：递归

class Solution {
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        sort(nums.begin(),nums.end());
        vector<vector<int>>  res;
        res.reserve(pow(2,nums.size()));
        vector<int> v; 
        subset(res,nums,0,v);
        return res;
    }
private:
    void subset(vector<vector<int>>& res,vector<int>& s,int start,vector<int>& v)
    {
        if(start == s.size())
        {
            res.push_back(v);
            return;
        }
        subset(res,s,start+1,v);
        v.push_back(s[start]);
        subset(res,s,start+1,v);
        v.pop_back(); 
    }
};