LeetCode 42 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.

这道题其实是道简单题，但是有几点要注意
1.这两个String开头并没有空格、“+”、“-”，也没有多余的‘0’；
2.这个String代表的是整数，意味着这两个字串的任意一个位置的字符为“0”~“9”之间的值，即没有“.”{代表的是小数}。
思路1：使用java的BigInteger

import java.math.BigInteger;

public class Solution {
    public String multiply(String num1, String num2) {
        BigInteger n1=new BigInteger(num1);
        BigInteger n2=new BigInteger(num2);
        return String.valueOf(n1.multiply(n2));
    }
}

思路2：模拟手算，但是对每一位及时加起来，注意我采用的是num2*num1

public class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0")||num2.equals("0")) return "0";
    	char [] temp=new char [num1.length()+num2.length()];
    	Arrays.fill(temp,'0');   	
    	StringBuilder sb=new StringBuilder(String.valueOf(temp));
    
    	for(int i=num1.length()-1;i>=0;i--){
			int carry=0;
			int a=num1.charAt(i)-'0';
    		for(int j=num2.length()-1;j>=0;j--){
    			int b=num2.charAt(j)-'0';
    			int value=carry+a*b+(sb.charAt(i+j+1)-'0');
    			carry=value/10;
    			value%=10;
    			sb.setCharAt(1+i+j, (char)(value+'0'));
    		}
    		sb.setCharAt(i, (char)(carry+'0'));
    	}
    	return sb.charAt(0)=='0'?sb.substring(1):sb.toString();
    }
}