[LeetCode]--43. Multiply Strings

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.

字符串拆分成字符，然后计算，注意进位就行，进位中又要特别注意最高位进位。最开始我用int计算，其实还是逃不过超出范围的限制，所以就用个int的数组，只记录每一位数字，然后用这个数组来构造字符串。

public class Solution {
    public String multiply(String num1, String num2) {
        int[] res = new int[num1