Subsets

Given a set of distinct integers, return all possible subsets.

Note

Elements in a subset must be in non-descending order.
The solution set must not contain duplicate subsets.

Example

If S = [1,2,3], a solution is:

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

 
class Solution {
public:
    /**
     * @param S: A set of numbers.
     * @return: A list of lists. All valid subsets.
     */
    vector<vector<int>> subsets(vector<int> &nums) {
        vector<vector<int>> output;
        sort(nums.begin(), nums.end());
        
        output.push_back(vector<int>());
        
        for(int n : nums) {
           for(int i = output.size()-1; i >=0; i--) {
               vector<int> current = output[i];
               current.push_back(n);
               output.push_back(current);
           }
        }
        
        return output;
    }
};

 
class Solution {
public:
   void helper(vector<int>&nums, int start, vector<int> & tmp, vector<vector<int>> &ans) {
       for(int i = start; i < nums.size(); i++) {
           tmp.push_back(nums[i]);
           ans.push_back(tmp);
           helper(nums, i+1, tmp, ans);
           tmp.pop_back();
       }
   }
   
    /**
     * @param S: A set of numbers.
     * @return: A list of lists. All valid subsets.
     */
    vector<vector<int>> subsets(vector<int> &nums) {
        sort(nums.begin(), nums.end());
        vector<vector<int>> ans;
        vector<int> tmp;
        helper(nums, 0, tmp, ans);
        ans.push_back(vector<int>());
        return ans;
    }
};

 
class Solution {
public:
   void helper2(vector<int> &S, int k, int current, vector<int> &tmp, vector<vector<int>> &ans) {
        if (k == 0) {
            ans.push_back(tmp);
            return;
        }
        
        for(int i = current; i <= S.size()-k; i++)  {
           tmp.push_back(S[i]);
           helper2(S, k-1, i + 1, tmp, ans);
           tmp.pop_back();
        }
    }
   
    void findSubsetsWithLen(vector<int> &S, int k, vector<vector<int>> &ans) {
        vector<int> tmp;
        helper2(S, k, 0, tmp, ans);
    }
    
    /**
     * @param S: A set of numbers.
     * @return: A list of lists. All valid subsets.
     */
    vector<vector<int>> subsets(vector<int> &S) {
        vector<vector<int>>  ans;
        sort(S.begin(), S.end());
        for(int i = 0; i <= S.size(); i++) {
            findSubsetsWithLen(S, i, ans);
        }
        return ans;
    }
};