private void StrReverse(ref String str, int left, int right)
{
    if (left >= right || right >= str.Length) return;
 
    for (int i = left, j = right; i < j; i++, j--)
    {
        Char tmp = str[i];
        this.ReplaceCharInString(ref str, i, str[j]);
        this.ReplaceCharInString(ref str, j, tmp);
    }
}