[Leetcode] Permutation Sequence

class Solution {
private:
    int a[10];
    bool used[10];
    string res;

public:
    string getPermutation(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        res = "";
        for (int i = 1; i <= n; ++i)
            used[i] = false;
        
        dfs(0, n, k);
        return res;
    }
    
    void dfs(int idx, int& n, int& k)
    {
        if (k == 0)
            return;
            
        if (idx == n)
        {
            --k;
            if (k == 0)
            {
                for (int i = 0; i < n; ++i)
                {
                    res += (char)(a[i] + '0');
                }
            }
        }
        
        for (int i = 1; i <= n; ++i)
        {
            if (!used[i])   
            {
                a[idx] = i;
                used[i] = true;
                            
                dfs(idx + 1, n, k);
                used[i] = false;
            }

        }
    }
};