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

 1 class Solution {
 2 public:
 3    vector<vector<int>> subsets(vector<int>& nums) {
 4       sort(nums.begin(),nums.end());
 5        vector<int> v;
 6        vector<vector<int>> res;
 7        res.push_back(v);
 8        for(int i=0;i<nums.size();i++)
 9        {
10            for(int j=res.size()-1;j>=0;j--)
11            {
12                v.assign(res[j].begin(),res[j].end());
13                v.push_back(nums[i]);
14                res.push_back(v);
15            }
16        }
17        return res;
18    }
19 };

方法二：递归

 1 class Solution {
 2 public:
 3     vector<vector<int>> subsets(vector<int>& nums) {
 4         sort(nums.begin(),nums.end());
 5         vector<vector<int>>  res;
 6         res.reserve(pow(2,nums.size()));
 7         vector<int> v; 
 8         subset(res,nums,0,v);
 9         return res;
10     }
11 private:
12     void subset(vector<vector<int>>& res,vector<int>& s,int start,vector<int>& v)
13     {
14         if(start == s.size())
15         {
16             res.push_back(v);
17             return;
18         }
19         subset(res,s,start+1,v);
20         v.push_back(s[start]);
21         subset(res,s,start+1,v);
22         v.pop_back(); 
23     }
24 };