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=;i<nums.size();i++)
        {
            for(int j=res.size()-;j>=;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(,nums.size()));
         vector<int> v;
         subset(res,nums,,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+,v);
         v.push_back(s[start]);
         subset(res,s,start+,v);
         v.pop_back();
     }
 };