Leetcode 784 Letter casePermutation

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class 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;
        
    } 
}
};