784. Letter Case Permutation

Problem:

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"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]

Note:

• S will be a string with length between 1 and 12.
• S will consist only of letters or digits.

思路：

Solution (C++):

vector<string> letterCasePermutation(string S) {
    vector<string> res;
    backtrack(S, 0, res);
    return res;
}
void backtrack(string& s, int i, vector<string>& res) {
    if (i == s.size()) {
        res.push_back(s);
        return;
    }
    backtrack(s, i+1, res);
    if (isalpha(s[i])) {
        s[i] ^= (1 << 5);
        backtrack(s, i+1, res);
    }
}

性能：

Runtime: 24 ms  Memory Usage: 9.6 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB