46. Permutations

class Solution {
private:
    void helper(vector<int>& nums, int idx, vector<vector<int>>& res) {
        if (idx + 1 == nums.size()) {
            res.push_back(nums);
            return;
        }
        for (int i = idx; i < nums.size(); i++) {
            swap(nums[i], nums[idx]);
            helper(nums, idx + 1, res);
            swap(nums[i], nums[idx]);
        }
        return;
    }
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> res;
        helper(nums, 0, res);
        return res;
    }
};