给定一个字符串S，通过将字符串S中的每个字母转变大小写，我们可以获得一个新的字符串。返回所有可能得到的字符串集合。

示例:
输入: S = “a1b2”
输出: [“a1b2”, “a1B2”, “A1b2”, “A1B2”]

输入: S = “3z4”
输出: [“3z4”, “3Z4”]

输入: S = “12345”
输出: [“12345”]

代码

class Solution {List<String> res=new ArrayList<>();public List<String> letterCasePermutation(String S) {StringBuilder stringBuilder=new StringBuilder();letter(stringBuilder,0,S);return res;}public void letter(StringBuilder stringBuilder,int loc,String s) {if(loc==s.length()) {res.add(stringBuilder.toString());return;}//到达边界if(Character.isDigit(s.charAt(loc))){//数字不能改变stringBuilder.append(s.charAt(loc));letter(stringBuilder, loc+1, s);stringBuilder.deleteCharAt(loc);} else {stringBuilder.append(s.charAt(loc));//没有翻转letter(stringBuilder, loc+1, s);stringBuilder.deleteCharAt(loc);//回溯stringBuilder.append(tan(s.charAt(loc)));//翻转letter(stringBuilder, loc+1, s);stringBuilder.deleteCharAt(loc);}}public char tan(char c) {if(c>='a'&&c<='z')return (char)(c-32);else if(c>='A'&&c<='Z')return (char)(c+32);return c;}
}