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