482. License Key Formatting

https://leetcode.com/contest/leetcode-weekly-contest-14/problems/license-key-formatting/

Solution:
feel like it is not a "medium" level question, should be a "simple" one. Scan from the right to left, if it is '-', remove it; if it is char from 'a' to 'z', convert it to the corresponding uppercase, count + 1; otherwise, count + 1, continue.  If the total elements accumulate to K, add '-'; continue scanning until end.

Code:

class Solution {
public:
    string licenseKeyFormatting(string S, int K) {
        int i = S.size()-1, t = 0;
        while(i>=0){
            if(S[i] == '-') S.erase(S.begin()+i);
            else{
                if(S[i] >= 'a' && S[i] <= 'z' ) S[i] = S[i] - 'a' + 'A';
                ++t;
                if(t == K){
                    S.insert(S.begin()+i, '-');
                    t = 0;
                }
            }
            --i;
        }
        if(S[0] == '-') S.erase(S.begin());
        return S;
    }
};