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):

1. "123"
2. "132"
3. "213"
4. "231"
5. "312"
6. "321"

Given n and k, return the kth permutation sequence.

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

$X = an(n - 1)!+a{n − 1}(n −2)!+…. + a_2\times 1!+a_1\times 0!$

$9!$ 并不大，所以我一开始写了个暴力，T 了，看来 case 数不是一般的多，那就只能找规律了。