leetcode 917. Reverse Only Letters

class Solution {
public:
    bool isString(char s)
    {
        if((s<='z'&&s>='a')||(s<='Z'&&s>='A'))
            return true;
        else return false;
    }
    string reverseOnlyLetters(string &S) {
        
        int l = S.length();
        int i=0,j=l-1;
        while(i<j)
        {
            while(i<j&&(!isString(S[i])))
            {
                i++;
            }
            while(i<j&&(!isString(S[j])))
            {
                j--;
            }
            char tmp = S[i];
            S[i] = S[j];
            S[j] = tmp;
            i++;
            j--;
        }
        return S;
    }
};