The set `[1, 2, ..., n]` has `n!` permutations. Listing them in order gives a sequence; given `n` and `k`, return the `k`-th permutation sequence (1-indexed) as a string.
Example 1
Input:n = 3, k = 3
Output:"213"
Explanation:Order: 123, 132, 213, ... the 3rd is 213.