LeetCode|Subsets

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Subsets

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],
  []
]

DFS+位运算
枚举每一个元素出现/不出现在subset中。因为用32位的unsigned int，所以最多能记录32个元素的集合，超过32个的话数目太多，题目肯定不会检验。

class Solution {
public:
    vector<vector<int> > subsets(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        vector<vector<int> > res;
        flag = 0;
        helper(nums, res, 0);
        return res;
    }
private:
    void helper(vector<int>& nums, vector<vector<int> >& res, int k){
        int  n = nums.size();
        if(k == n){
            vector<int> tmp;
            for(unsigned int i = 0, b = 1; i < n; i++){
                if(b&flag) tmp.push_back(nums[i]);
                b <<= 1;
            }
            res.push_back(tmp);
            return;
        }
        helper(nums, res, k+1);
        flag |= (1<<k);
        helper(nums, res, k+1);
        flag ^= (1<<k);
    }
    unsigned int flag;
};