License Key Formatting

Given a string S that consists of only alphanumeric characters and dashes. The string is separated into N + 1 groups by N dashes. Also given an integer K.

We want to reformat the string S, such that each group contains exactly K characters, except for the first group, which could be shorter than K but still must contain at least one character. Furthermore, there must be a dash inserted between two groups, and you should convert all lowercase letters to uppercase.

Return the reformatted string.

• Time : O(n)
• Space : O(1)

class Solution {
    char convert(char ch) {
        if('0' <= ch and ch <= '9') return ch;
        if('A' <= ch and ch <= 'Z') return ch;
        return ch - 'a' + 'A';
    }
public:
    string ReFormatString(string S, int K) {
        string res = "";
        for (int i = S.length() - 1, split = 0; i >= 0; i--) {
            if (S[i] == '-') continue;
            res.push_back(convert(S[i]));
            if (res.length() - split == K) {
                res.push_back('-');
                split = res.length();
            }
        }
        if (res.back() == '-') res.pop_back();
        reverse(begin(res), end(res));
        return res;
    }
};