Add Strings
A easy-tier problem at 52% community acceptance, tagged with Math, String, Simulation. Reported in interviews at Wayfair and 9 others.
Add Strings looks trivial until you realize you can't just convert to integers and add them. It's been asked at Airbnb, Meta, Microsoft, and TikTok. The trap is thinking you need built-in math functions. The actual skill they're testing is whether you can simulate grade-school addition by hand: iterate through both strings backwards, handle the carry, build the result. Half the candidates who see this problem get it wrong, usually by trying a shortcut that fails on edge cases or large numbers. If this problem hits your live assessment and you blank on the simulation pattern, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "Add Strings"
Add Strings 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 because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe trick is to stop thinking like a programmer and think like a ten-year-old adding on paper. Start at the rightmost digit of each string, add them plus any carry, write down the last digit of that sum, pass the carry left. Keep going until both strings are consumed and there's no carry left. Most people overthink it: they try to reverse the strings, use list comprehensions, or reach for string methods that don't exist. The actual solution is a single loop with a carry variable. Edge cases are deceptively simple once you see the pattern: strings of different lengths, empty strings, a final carry that needs a new digit. This is classified as Math, String, and Simulation. The Math tag is a red herring. Simulation is the real category. StealthCoder is your hedge if you freeze on whether to iterate forward or backward, or if you second-guess the carry logic under time pressure.
Pattern tags
You know the problem.
Make sure you actually pass it.
Add Strings 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 because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Add Strings interview FAQ
Is Add Strings actually hard or is it just asked at big companies?+
It's genuinely easy in retrospect, but the acceptance rate is 52%, which suggests many candidates fail in the live setting. The difficulty isn't the algorithm. It's staying calm and not trying to use built-in math functions. Wayfair, Meta, and Microsoft have all asked it, so they expect a clean simulation.
What's the most common mistake on this problem?+
Trying to convert to integers. The problem exists specifically to test whether you can simulate addition without relying on the language's integer type. Another trap: forgetting the final carry. If both strings are exhausted but carry is still 1, you need one more digit in the result.
How does Add Strings relate to big integer problems?+
It's the foundation. If you can't simulate addition on strings, you can't do multiplication, subtraction, or division on big integers. This problem teaches the core pattern. It appears across 10 companies, many of whom also ask harder math-on-strings variants.
Can I solve this without reversing the strings?+
Yes. Use two pointers starting from the end (length minus one) and work backwards through both strings. Append each digit to the result, then reverse the result at the end. Some people prefer to build the answer in reverse and flip it once, others use integer indices. Both work.
Is this problem still asked in 2024 or is it outdated?+
Still active. It's on the list for Airbnb, Meta, Microsoft, TikTok, and others. It's not a trend problem. It's a stable filter for whether you can code basic algorithms without reaching for library functions. Don't skip it.
Want the actual problem statement? View "Add Strings" on LeetCode →