415. Add Strings - LeetCode Solution

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        ptr1 = len(num1) - 1
        ptr2 = len(num2) -1

        carry = 0
        res = ""

        while ptr1 >=0 or ptr2 >= 0:

            if ptr1 < 0:
                a = str( int(num2[ptr2]) + carry)
            elif ptr2 <0:
                a = str(int(num1[ptr1]) + carry)
            else:
                a = str(int(num1[ptr1]) + int(num2[ptr2]) + carry)


            if len(a) == 2:
                carry = int(a[0])
                res += a[1]
            else:
                carry= 0
                res+=a

            if ptr2 >=0:
                ptr2-=1
            if ptr1>=0:
                ptr1-=1
        
        
        if carry!= 0:
            res+= str(carry)

        return res[::-1]