python leetcode 43. Multiply Strings

class Solution(object):
    def multiply(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        ln1=len(num1)
        ln2=len(num2)
        num1,num2=num1[::-1],num2[::-1]
        arr = [0]*(ln1+ln2)
        ans=''
        for i in range(ln1):
            for j in range(ln2):
                arr[i+j]+=int(num1[i])*int(num2[j])
        for i in range(ln1+ln2):
            digit = arr[i] % 10
            f = arr[i] / 10 
            if f >0:
                arr[i+1] += f
            ans+=str(digit)
        ans=ans[::-1]
        i=0
        while ans[i]=='0' and i < len(ans)-1:
            i+=1
        return ans[i:]