[LeetCode] Subsets II

题目描述:

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],
  []
]

解题思路：

回溯法

class Solution {
public:
    vector<vector<int>> subsetsWithDup(vector<int>& nums) {
        vector<vector<int>> result;
        if (nums.size() == 0) {
            return result;
        }
        
        sort(nums.begin(), nums.end());
        vector<int> elem;
        
        for (int i = 0; i <= nums.size(); ++i) {
            subsets(nums, i, 0, result, elem);
        }
        
        return result;
    }
private:
    void subsets(const vector<int> &nums, int len, int cur, vector<vector<int>> &result, vector<int> &elem) {
        if (elem.size() == len) {
            result.push_back(elem);
            return;
        }
        
        for (int i = cur; i < nums.size(); ++i) {
            elem.push_back(nums[i]);
            subsets(nums, len, i + 1, result, elem);
            elem.pop_back();
            // 跳过重复的元素
            for (int j = i + 1; j < nums.size(); ++j) {
                if (nums[j] != nums[i]) {
                    break;
                }
                ++i;
            }
        }
    }
};