leetcode Permutation Sequence

class Solution {
public:
string getPermutation(int n, int k) {
string str = string("123456789").substr(0,n);
string res(n,' ');
for(int i = 0; i < n; ++i)
res[i] = getPermutationHelper(str, k);
return res;
}

char getPermutationHelper(string &str, int &k)
{
int temp = factorial(str.size()-1);
int i = (k-1)/temp;
char res = str[i];
str.erase(i,1);
k = k - i*temp;
return res;
}

int factorial(int n)
{
int res = 1;
for(int i = 2; i <= n; ++i)
res *= i;
return res;

}

};