Permutation Sequence

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.

Here I use a vector to record the remaining digits. The current factorial function is not optimized... it should be fine since n is between 1 and 9.

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