Leetcode 78. Subsets

题目

链接：https://leetcode.com/problems/subsets/

**Level: ** Medium

Discription:
Given a set of distinct integers, nums, return all possible subsets (the power set).

Example 1:

Input: nums = [1,2,3]
Output:
[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

Note:

• The solution set must not contain duplicate subsets.

代码

class Solution {
public:
    void dfs(vector<int>& nums, vector<int>& temp, vector<vector<int>>& ret,int n, int index)
    {
        if(n == temp.size())
        {
            ret.push_back(temp);
            return;
        }
                       
        for(int i=index;i<nums.size();i++)
        {
            if((n-temp.size()) > (nums.size()-i))
                return;
            temp.push_back(nums[i]);
            dfs(nums, temp, ret, n, i+1);
            temp.pop_back();
        }
            
    }
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<int> temp;
        vector<vector<int>> ret;
        for(int i=0;i<=nums.size();i++)
        {
            dfs(nums, temp, ret, i, 0);
        }
        return ret;
    }
};

思考

• 算法时间复杂度为O(\(2^n\))，指数级的复杂度一般在求全部解的问题中会出现，空间复杂度为O(n)。
• 求n个元素的所有子集。