(46)Permutations

class Solution {
public:
    vector<vector<int>> ret;
    vector<int> seq;
    bool used[6] = {0};
    int n;
    vector<vector<int>> permute(vector<int>& nums) {
        n = nums.size();
        seq.resize(n);
        backtrack(nums, 0);
        return ret;
    }

    void backtrack(vector<int> &nums, int idx)
    {
        if (idx >= n) 
        {
            ret.emplace_back(seq);
            return;
        }
        for (int i = 0; i < n; ++i)
        {
            if (used[i]) continue;
            seq[idx] = nums[i], used[i] = true;
            backtrack(nums, idx + 1);
            used[i] = false;
        }      
    }
};