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.

public class Solution {
    public String multiply(String num1, String num2) {
        StringBuilder ans = new StringBuilder();
        int[] tmp = new int[num1.length() + num2.length()];
        for(int i = num1.length()-1; i>=0;i--){
            for(int j = num2.length()-1; j>=0;j--){
                tmp[i+j+1] += (num1.charAt(i) - '0')*(num2.charAt(j) - '0');
            }
        }
        
        for(int i = tmp.length-1; i >= 1;i--){
            tmp[i-1] += tmp[i]/10;
            tmp[i] %= 10;
        }
        
        int left = 0;
        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。
        for(;left < tmp.length; left++){
            ans.append(tmp[left]);
        }
        return ans.toString();
    }
}