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); } } };