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23class Solution {
public:
vector<string> letterCasePermutation(string S) {
vector<string> ss;
dfs(S,0,ss);
return ss;
}
private:
void dfs(string &s,int k,vector<string> &ss){
if(k==s.length()) {
ss.push_back(s);//!!
return;
}
dfs(s,k+1,ss);//!!
if(!isalpha(s[k])) return;
else{
s[k]^=1<<5;
dfs(s,k+1,ss);
s[k]^=1<<5;
}
}
};