原题链接：https://leetcode.com/problems/letter-case-permutation/

class Solution {
public:
    vector<string> res;
    vector<string> letterCasePermutation(string s) {
        dfs(s, 0);
        return res;
    }
    
    void dfs(string s, int i) {
        if (i == s.size()) {
            res.push_back(s);
            return;
        }
        
        dfs(s, i + 1);
        
        if (s[i] >= 'A') {
            s[i] ^= 32;
            dfs(s, i + 1);
        }
    }
};