leetcode解题: Subsets (78)

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Given a set of distinct integers, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example,
If nums = [1,2,3], a solution is:

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[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

解法1： Backtracking O(2^N)

Backtracking的模板解法。 用一个pos记录当前扫描到的位置。对于每一种情况，都是一个subset，所以递归的时候一开始就要把当前的结果给保存到结果集中。
C++

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class Solution {
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<vector<int>> res;
        if (nums.empty()) {
            return res;
        }
        
        vector<int> cur;
        helper(nums, 0, cur, res);
        return res;
    }
    
    void helper(vector<int>& nums, int pos, vector<int>& cur, vector<vector<int>>& res) {
        
        res.push_back(cur);
        
        for (int i = pos; i < nums.size(); ++i) {
            cur.push_back(nums[i]);
            helper(nums, i + 1, cur, res);
            cur.pop_back();
        }
        
    }
};

Java

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