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。

 1 class Solution {
 2 public:
 3     void help(vector<vector<int> >& res, vector<int>& nums, vector<int> cand, int cur)
 4     {
 5         if (cur == nums.size())
 6         {
 7             res.push_back(cand);
 8             return;
 9         }
10         int nex, n = nums.size();
11         for (nex = cur + 1; nex < n && nums[nex] == nums[cur]; nex++);
12         help(res, nums, cand, nex);
13         for (int i = cur; i < nex; i++)
14         {
15             cand.push_back(nums[i]);
16             help(res, nums, cand, nex);
17         }
18     }
19     vector<vector<int>> subsetsWithDup(vector<int>& nums) {
20         sort(nums.begin(), nums.end(), less<int>());
21         vector<int> cand;
22         vector<vector<int> > res;
23         help(res, nums, cand, 0);
24         return res;
25     }
26 };