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"] Inpu

test.cpp

class Solution {
public:
    vector<string> letterCasePermutation(string S) {
        vector<string> vs;
        helper(vs,S,0);
        return vs;
    }
    
    void helper(vector<string> & vs,string &S,int p){
        if(p == S.size()){
            vs.push_back(S);
            return;
        }
        
        if(S[p] >= '0' && S[p] <= '9')
            helper(vs,S,p+1);
        else if(S[p] >= 'a' && S[p] <= 'z'){
            helper(vs,S,p+1);
            S[p] += 'A' - 'a';
            helper(vs,S,p+1);
        }
        
        else if(S[p] >= 'A' && S[p] <= 'Z'){
            helper(vs,S,p+1);
            S[p] += 'a' - 'A';
            helper(vs,S,p+1);
        }
    }
};