Subsets II -- LeetCode

Given a collection of integers that might contain duplicates, 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 = [1,2,2], a solution is:

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

思路：我们先考虑所有数字不重复出现的情况。那么所有子集的个数是2^n，即每个数字都只有出现或者不出现这2种情况。而当数字重复出现时，我们将该数字视为一种特殊的数字。比如说输入中含有2个5，那么我们就有3种选择：不选5，选1个5，选2个5。

class Solution {
public:
    void help(vector<vector<int> >& res, vector<int>& nums, vector<int> cand, int cur)
    {
        if (cur == nums.size())
        {
            res.push_back(cand);
            return;
        }
        int nex, n = nums.size();
        for (nex = cur + 1; nex < n && nums[nex] == nums[cur]; nex++);
        help(res, nums, cand, nex);
        for (int i = cur; i < nex; i++)
        {
            cand.push_back(nums[i]);
            help(res, nums, cand, nex);
        }
    }
    vector<vector<int>> subsetsWithDup(vector<int>& nums) {
        sort(nums.begin(), nums.end(), less<int>());
        vector<int> cand;
        vector<vector<int> > res;
        help(res, nums, cand, 0);
        return res;
    }
};