given two numbers represented as strings, return multiplication of the numbers as a string.
note:
the numbers can be arbitrarily large and are non-negative.
converting the input string to integer is not allowed.
you should not use internal library such as biginteger.
字元串拆分成字元，然後計算，注意進位就行，進位中又要特别注意最高位進位。最開始我用int計算，其實還是逃不過超出範圍的限制，是以就用個int的數組，隻記錄每一位數字，然後用這個數組來構造字元串。
還有一個比我簡便一點點方法。