784. Letter Case Permutation

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"]

把字符串里的字母任意改为大写或小写，求能获得的所有字符。

我的解决思路是一个个遍历字符，碰到数字直接加上，碰的字母的，就把已经有的字符串复制一遍，前面一半字符串尾补加小写字母，后面一半字符串尾补加大写字母。

class Solution {
public:
    vector<string> letterCasePermutation(string S) {
        vector<string> res(1);
        for (int i=0; i<S.length(); ++i) {
            char c = S[i];
            int len = res.size();
            if ((c>='a' && c <='z') || (c>='A' && c<='Z')) {
                for (int j=0; j<len; ++j) {
                    res.push_back(res[j]);
                    res[j].push_back(tolower(c));
                    res[j+len].push_back(toupper(c));
                }
            }
            else
                for (int j=0; j<len; ++j)
                    res[j].push_back(c);
        }
        return res;
    }
};