leetcode_Multiply Strings

描述：

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.

思路：

简而言之，要实现的就是BigInteger(a).Multiply(BigInteger(b))的功能，但很显然，leetcode中不让用BigInteger

代码：

public class Solution {
 public  String multiply(String num1, String num2)
	{
		if(num1==null||num2==null)
			return new String();
		num1=num1.trim();
		num2=num2.trim();
		if(num1.equals("0")||num2.equals("0"))
			return "0";
	   List<Integer>listNum1=new ArrayList<>();
	   List<Integer>listNum2=new ArrayList<>();
	   List<Integer>listResult=new ArrayList<>();
	   int len1=num1.length();
	   int len2=num2.length();
	   int i=0,j=0,lenResult=0,index=0;
	   int baseNum=0,flowNum=0,tempNum1=0,tempNum2=0;
	   for( i=len1-1;i>=0;i--)
		   listNum1.add(num1.charAt(i)-'0');
	   for( i=len2-1;i>=0;i--)
		   listNum2.add(num2.charAt(i)-'0');
	   tempNum2=listNum2.get(0);
	   for(i=0;i<len1;i++)
	   {
		   tempNum1=listNum1.get(i);
		   tempNum1=tempNum1*tempNum2+flowNum;
		   baseNum=tempNum1%10;
		   flowNum=tempNum1/10;
		   listResult.add(baseNum);
	   }
	   if(flowNum!=0)
	   {
		   listResult.add(flowNum);
		   flowNum=0;
	   }
	   for(j=1;j<len2;j++)
	   {
		   baseNum=0;flowNum=0;
		   tempNum2=listNum2.get(j);
		   lenResult=listResult.size();
		   for(i=0;i<len1;i++)
		   {
			   index=i+j;
			   if(index<lenResult)
			   {
				   tempNum1=listNum1.get(i);
				   tempNum1=tempNum1*tempNum2+flowNum+listResult.get(index);
				   baseNum=tempNum1%10;
				   flowNum=tempNum1/10;
				   listResult.set(index, baseNum);
			   }
			   else {
				   tempNum1=listNum1.get(i);
				   tempNum1=tempNum1*tempNum2+flowNum;
				   baseNum=tempNum1%10;
				   flowNum=tempNum1/10;
				   listResult.add(baseNum);
			   }
		   }
		   if(flowNum!=0)
		   {
			   listResult.add(flowNum);
			   flowNum=0;
		   }
			   
	   }
	   if(flowNum!=0)
		   listResult.add(flowNum);
	   StringBuilder sBuilder=new StringBuilder();
	   for(int num:listResult)
		   sBuilder.append(num);
	   sBuilder.reverse();
	   return sBuilder.toString();
	   
	}
}