Add Strings

Add Strings

What is this problem about?

The Add Strings interview question asks you to perform basic arithmetic addition on two large numbers provided as strings. You cannot convert the strings directly into integers using built-in high-level functions (like BigInteger or int() in Python for very large numbers). The goal is to return the sum of these two strings as a string.

Why is this asked in interviews?

Companies like Meta and Amazon use the Add Strings coding problem to see if a candidate understands how numbers are represented and processed at a low level. It tests your ability to handle strings of arbitrary length, manage "carries" during addition, and avoid integer overflow errors that occur when numbers exceed the standard 64-bit limit.

Algorithmic pattern used

This problem follows the Math and Simulation interview pattern. Specifically, it simulates the "column addition" we learn in primary school. You start from the last characters of both strings (the ones place), add the digits along with any carry from the previous step, and proceed toward the front of the strings.

Example explanation

Imagine adding "456" and "77".

1. Step 1: Add the last digits: $6 + 7 = 13$. Write down 3, carry 1.
2. Step 2: Add the next digits plus carry: $5 + 7 + 1 = 13$. Write down 3, carry 1.
3. Step 3: Add the next digit plus carry: $4 + 0 + 1 = 5$. Write down 5.
4. Final Result: Combine the digits to get "533".

Common mistakes candidates make

• Direct Conversion: Trying to convert the entire string to a number. If the string has 500 digits, it will crash or lose precision.
• Forgetting the Final Carry: After the loop ends, a carry might still remain (e.g., $99 + 1$). Failing to append this last 1 is a frequent error.
• String Concatenation in a Loop: In some languages, strings are immutable. Repeatedly adding to the front of a string can result in $O(N^2)$ complexity. Using a StringBuilder or list is preferred.

Interview preparation tip

Always handle the strings from right to left using two pointers. This mirrors how we naturally add numbers and makes the logic for differing string lengths much easier to manage.