Given a string S, we can transform every letter individually to be lowercase or uppercase to create another
string. Return a list of all possible strings we could create.
Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]
Input: S = "3z4"
Output: ["3z4", "3Z4"]
Input: S = "12345"
Output: ["12345"]
Note:
S will be a string with length between 1 and 12.
S will consist only of letters or digits.
public List<String> letterCasePermutation(String S) {
List<String> result = new ArrayList<>();
generate(result, S, "");
return result;
}
public void generate(List<String> result, String s, String buffer) {
if(buffer.length() == s.length()) {
result.add(buffer);
return;
}
int i = buffer.length();
char c = s.charAt(i);
generate(result, s, buffer + c);
if(Character.isLetter(c)) {
String next = buffer;
if(c >= 'a' && c <= 'z') {
next += Character.toUpperCase(c);
} else {
next += Character.toLowerCase(c);
}
generate(result, s, next);
}
}