[LeetCode] Multiply Strings

Problem:

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:O(n*m)

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

Key Concept: We can use an array to store each single multiply result