482. License Key Formatting

static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

class Solution 
{
public:
    string licenseKeyFormatting(string S, int K) 
    {
        int len=S.length();
        int count=0;
        string res;
        for(int i=len-1;i>=0;i--)
        {
            char c=toupper(S[i]);
            if(c=='-')
                continue;
            res.push_back(c);
            count++;
            if(count==K)
            {
                res.push_back('-');
                count=0;
            }           
        }
        if(res.back()=='-')
            res.pop_back();
        reverse(res.begin(),res.end());
        return res;
    }
};

从后往前扫描，问题不大