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(,'a');        // string to add 'a'
         unordered_set<char> vowel={'a','e','i','o','u','A','E','I','O','U'};
         int front=;
         int len=S.size();
         int alen=;
         string res;
         for(int i=;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[])==vowel.end())
                 {
                     cur.push_back(cur[]);
                     cur.erase(cur.begin());
                 }
                 res=res+cur+"ma"+a.substr(,alen)+" ";
                 alen++;
                 front=i+;
             }
         }
         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