Here is my C++ solution

// helper function
bool isPalindrome ( const string & str)
{
    for (int i = 0; i < str.length() / 2; i++)
    { 
        if (str[i] != str[str.length() - i - 1])
            return false;
    }
    return true;
} 

int  palindromeIndex(string& s)
{
  int backward = 0, forward = s.size() - 1;

  bool bPallindrome = true;

  while (backward < forward) 
  {
      if (s[backward] != s[forward]) 
      {
          bPallindrome = false;
          break;
      }
      backward++;
      forward--;
  }
  int result = -1;

  if (!bPallindrome)
  {
    result = isPalindrome(s.erase(forward, 1)) ? forward: backward;
  }
  return result;

}