[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，
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，
