Leetcode-917 Reverse Only Letters(仅仅反转字母)

class Solution
{
    public:
        string reverseOnlyLetters(string S)
        {
            string onlyLetter;
            for(auto c:S)
            {
                if(isalpha(c))
                    onlyLetter += c;
            }
            reverse(onlyLetter.begin(),onlyLetter.end());
            for(int i = 0,j = 0;i < S.size();i ++)
            {
                if(isalpha(S[i]))
                {
                    S[i] = onlyLetter[j++];
                }
            }
            return S;
        }
};