LeetCode-Permutation SequencePermutation Sequence

class Solution {
public:
string getPermutation(int n, int k) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<int> A(n + 1);
string ans;
A[0] = 1;
for (int i = 1; i < A.size(); ++i)
{
A[i] = i * A[i - 1];
}
vector<int> B(n);
for (int i = 0; i < B.size(); ++i)
{
B[i] = i + 1;
}
for (int i = n - 1; i >= 0; --i)
{
if (k == 1)
{
for (int j = 0; j < B.size(); ++j)
{
ans += B[j] + '0';
}
break;
}
else
{
//根据k值计算当前应是哪个数
int tmp = (k - 1) / A[i];
vector<int>::iterator iter = B.begin() + tmp;
ans += *iter + '0';
//去除已选出的数，并更新k
B.erase(iter);
k -= tmp * A[i];
}
}
return ans;
}
};