Add Binary
A easy-tier problem at 56% community acceptance, tagged with Math, String, Bit Manipulation. Reported in interviews at SIG and 10 others.
Add Binary looks deceptively simple. You're adding two binary strings, returning their sum as a binary string. Apple, Google, Amazon, and Microsoft all ask it. The acceptance rate hovers around 56 percent, which means more than 40 percent of candidates either time out, mishandle the carry logic, or don't realize when string reversal is a trap. It's a warm-up problem that catches people who code on autopilot. If this hits your live assessment and you blank on the iteration order or carry propagation, StealthCoder surfaces a clean solution invisible to the proctor.
Companies that ask "Add Binary"
Add Binary is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code.
Get StealthCoderThe trick is that you can't just convert to integers, add them, and convert back. That fails on large binary strings that exceed your language's integer size. You have to iterate from the rightmost character of both strings backward, track the carry bit, and append results to a result string as you go. Most people get the carry logic right but then forget to reverse the result string at the end, or they reverse when they shouldn't depending on their build strategy. The real gotcha is the final carry: if you exit the loop and carry is still 1, you need to prepend it. This problem touches Math, String, Bit Manipulation, and Simulation all at once, which is why it survives interviews at scale. StealthCoder hedges the case where you second-guess yourself on the build direction during screen share.
Pattern tags
You know the problem.
Make sure you actually pass it.
Add Binary recycles across companies for a reason. It's easy-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Add Binary interview FAQ
Is Add Binary still asked at big tech?+
Yes. Google, Amazon, Microsoft, Meta, and Apple all report it in their interview signal. It's a staple warm-up at 11 major companies. The 56 percent acceptance rate means it's harder than it looks. If you see it in your assessment, take it seriously.
What's the main failure mode?+
Forgetting the final carry or reversing the result string incorrectly. Many candidates build left-to-right, add the carry, then reverse. Others iterate right-to-left and prepend. Both work, but mixing them breaks the solution silently during the assessment.
Should I convert to int, add, convert back?+
No. It fails on large inputs that exceed your language's integer bit width. Always iterate both strings right-to-left, handle carry manually, and build the result incrementally. That's the only safe approach in an interview.
How does this relate to Bit Manipulation?+
You're manipulating binary at the string level, not bitwise operators. The carry propagation is a core bit manipulation concept. Understanding how addition works at the binary level is the real skill being tested, not XOR or AND tricks.
Is this problem worth drilling before my OA?+
Yes. It's asked across 11 companies and has a 56 percent acceptance rate. Twenty minutes of practice on carry logic and string reversal pays off. If you do hit it live and blank, StealthCoder is your safety net.
Want the actual problem statement? View "Add Binary" on LeetCode →