LeetCode Permutation Sequence

题目：

The set

[1,2,3,…,n]

contains a total of n!
unique permutations.

By listing and labeling all of the permutations in order,

We get the following sequence (ie, for n = 3):

"123"

"132"

"213"

"231"

"312"

"321"

Given n and k, return the kth permutation sequence.

Note: Given n will be between 1 and 9 inclusive.

class Solution {
public:
string getPermutation(int n, int k) {
vector<int> num(n);
int f = 1;
for (int i = 0; i < n; i++) {
num[i] = i + 1;
f *= num[i];
}
k--;
string ans;
for (int i = 0; i < n; i++) {
f /= n - i;
int idx = k / f;
ans.push_back('0'+ num[idx]);
for (int j = idx; j < n - i - 1; j++)
num[j] = num[j + 1];
k %= f;
}
return ans;
}
};