class Solution {
List<String> resList;
public List<String> letterCasePermutation(String s) {
resList = new ArrayList<>();
StringBuilder sb = new StringBuilder(s);
backTrack(s, sb, 0);
return resList;
}
void backTrack(String s, StringBuilder sb, int index){
if(index==s.length()){
resList.add(sb.toString());
return;
}
if(Character.isAlphabetic(s.charAt(index))){
if(Character.isUpperCase(s.charAt(index)))
sb.setCharAt(index, Character.toLowerCase(s.charAt(index)));
else
sb.setCharAt(index, Character.toUpperCase(s.charAt(index)));
backTrack(s,sb,index+1);
sb.setCharAt(index, s.charAt(index));
}
backTrack(s,sb,index+1);
}
}
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