Write a method to compute all permutations of a string of unique characters.

Example1:

 Input: S = "qwe"
 Output: ["qwe", "qew", "wqe", "weq", "ewq", "eqw"]

Example2:

 Input: S = "ab"
 Output: ["ab", "ba"]

Note:

1. All charaters are English letters.
2. 1 <= S.length <= 9