43. Multiply Strings java solutions

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.

 1 public class Solution {
 2     public String multiply(String num1, String num2) {
 3         StringBuilder ans = new StringBuilder();
 4         int[] tmp = new int[num1.length() + num2.length()];
 5         for(int i = num1.length()-1; i>=0;i--){
 6             for(int j = num2.length()-1; j>=0;j--){
 7                 tmp[i+j+1] += (num1.charAt(i) - '0')*(num2.charAt(j) - '0');
 8             }
 9         }
10         
11         for(int i = tmp.length-1; i >= 1;i--){
12             tmp[i-1] += tmp[i]/10;
13             tmp[i] %= 10;
14         }
15         
16         int left = 0;
17         while(left < tmp.length-1 && tmp[left] == 0)left++;// 这里需要考虑一个 num1 和 num2 有一个为0 的情况。
18         for(;left < tmp.length; left++){
19             ans.append(tmp[left]);
20         }
21         return ans.toString();
22     }
23 }