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.

solution:直接乘会溢出，需要一位一位的乘。

 用一个数组d［］装所有一位乘积。

正确算法：

public class Solution {
    public String multiply(String num1, String num2) {
   
        String s1=new StringBuilder(num1).reverse().toString();
        String s2=new StringBuilder(num2).reverse().toString();
        int[] d= new int[s1.length()+s2.length()];
        for(int i=0;i<s1.length();i++){
            for(int j=0;j<s2.length();j++){
                d[i+j]=d[i+j]+(s1.charAt(i)-'0')*(s2.charAt(j)-'0');
            }
        }
        StringBuilder sb=new StringBuilder();
        int carry;
        int digit;
        for(int temp=0;temp<d.length;temp++){
            digit=d[temp]%10;
            carry=d[temp]/10;
            sb.insert(0,digit);
            if(temp<d.length-1)
            d[temp+1]=d[temp+1]+carry;
        }
    
        while(sb.length()>0 && sb.charAt(0)=='0'){
            sb.deleteCharAt(0);
        }
       if(sb.length()==0){
            return "0";
        }
        return sb.toString();
    }
}