824. Goat Latin

class Solution 
{
public:
    string toGoatLatin(string S) 
    {
        S.push_back(' ');         //add a space that the loop can deal with the last word
        string a(150,'a');        // string to add 'a'
        unordered_set<char> vowel={'a','e','i','o','u','A','E','I','O','U'};
        int front=0;
        int len=S.size();
        int alen=1;
        string res;
        for(int i=0;i<len;i++)      //once we meet a space ,we can deal with the word before it
        {
            if(S[i]==' ')
            {
                string cur=S.substr(front,i-front);
                if(vowel.find(cur[0])==vowel.end())      
                {
                    cur.push_back(cur[0]);
                    cur.erase(cur.begin());
                }        
                res=res+cur+"ma"+a.substr(0,alen)+" ";
                alen++;
                front=i+1;
            }
        }
        res.pop_back();    //delete the extra space
        return res;
    }
};

把原字符串剪断，一个单词一个单词处理，再把单词拼接成结果即可