[Interview] string permutation

Give a string, which only contains a-z. List all the permutation of upcase and lowcase.

For example, str = "ab",  the output should be
"ab", "aB", "Ab", "AB"

for str = "abc", the output should be
"abc", "abC", "aBc", "aBC", "Abc", "AbC", "ABc", "ABC"


[Thoughts]
首先，