0415.Add_Strings.py

"""
Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.

You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.

 

Example 1:

Input: num1 = "11", num2 = "123"
Output: "134"
Example 2:

Input: num1 = "456", num2 = "77"
Output: "533"
Example 3:

Input: num1 = "0", num2 = "0"
Output: "0"
 

Constraints:

1 <= num1.length, num2.length <= 104
num1 and num2 consist of only digits.
num1 and num2 don't have any leading zeros except for the zero itself.
"""

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        res = ""
        i, j = len(num1) - 1, len(num2) - 1
        carry = 0
        
        while i >= 0  and j >= 0:
            curr = int(num1[i]) + int(num2[j]) + carry
            carry = curr // 10
            single = curr % 10
            res += str(single)
            i -= 1
            j -= 1
            
        while i >= 0:
            curr = int(num1[i]) + carry
            carry = curr // 10
            single = curr % 10
            res += str(single)
            i -= 1
            
        while j >= 0:
            curr = int(num2[j]) + carry
            carry = curr // 10
            single = curr % 10
            res += str(single)
            j -= 1
            
        if carry != 0:
            res += str(carry)
            
        return res[::-1]