Generate binary string

Given a string containing of 0, 1 and ? - a wildcard character, generate all distinct binary strings that can be formed by replacing each wildcard character by either 0 or 1.

• Time : O(2^n)
• Space : O(2^n)

class Solution {
    void helper(vector<string>& res, string& s, int p) {
        if(p == s.length()) res.push_back(s);
        else {
            if(s[p] == '?') {
                s[p] = '0';
                helper(res,s,p + 1);
                s[p] = '1';
                helper(res,s,p + 1);
                s[p] = '?';
            } else helper(res, s, p + 1);
        }
    }
public:
    vector<string> generate_binary_string(string s) {
        vector<string> res;
        helper(res, s, 0);
        return res;
    }
};