【leetcode】仅仅反转字母

char * reverseOnlyLetters(char * S){
    int len = strlen(S);
    int left=0,right=len-1;
    char ch;
    while(left<right)
    {
        if (isalpha(S[left]) && isalpha(S[right]))
        {
            ch = S[left];
            S[left++] = S[right];
            S[right--] = ch;
        }
        while(left<right && !isalpha(S[left])) left++;
        while(left<right && !isalpha(S[right])) right--;
    }
    return S;
}