# 46. Permutations (Medium)

## 1.問題 

* 給予一個不重複數字的集合, 回傳所有可能的排列組合

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LHx7z8LhEk2sxMCHPDw%2F-LHx8ZXUKAjHwX1XU5Fj%2F201872103.jpg?alt=media\&token=8f9d863a-8cc3-448e-81cf-1dd2c176005e)

## 2.想法 

* Recursive + back tracking

## 3.程式碼

```
class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> res;
        vector<int> record;
        vector<bool> used(nums.size(), false);
        helper(res, nums, record, used);
        return res;
    }
private:
    void helper(vector<vector<int>>& res, vector<int>& nums, vector<int>& record, vector<bool>& used) {
        if (record.size() == nums.size()) {
            res.push_back(record);
            return;
        }
        for (int i = 0 ; i < nums.size(); i++) {
            if (!used[i]) {
                used[i] = true;
                record.push_back(nums[i]);
                helper(res, nums, record, used);
                record.pop_back();
                used[i] = false;
            }
        }
    }
};
```

## 4.Performance

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LHx7z8LhEk2sxMCHPDw%2F-LHx9Jlio2V-jrLfFCfD%2F201872104.jpg?alt=media\&token=909272df-c998-459e-beec-281adf939247)