09. Add Strings.cpp

/*
Add Strings
===========

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
{
public:
    string addStrings(string num1, string num2)
    {
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());

        string ans = "";
        int carry = 0, i = 0;

        while (i < num1.size() || i < num2.size() || carry)
        {
            int a = (i < num1.size() ? num1[i] - '0' : 0);
            int b = (i < num2.size() ? num2[i] - '0' : 0);

            int sum = a + b + carry;
            int digit = sum % 10;

            ans.push_back(digit + '0');
            carry = sum / 10;

            i++;
        }

        reverse(ans.begin(), ans.end());
        return ans;
    }
};