Expression Add Operators
A hard-tier problem at 42% community acceptance, tagged with Math, String, Backtracking. Reported in interviews at Pinterest and 3 others.
Expression Add Operators hits your assessment from Google, Meta, LinkedIn, or Pinterest, and it's brutal if you don't see the pattern. You get a string of digits and a target number. Your job: insert plus, minus, and multiply operators between digits to hit that target. The hard part isn't coding it, it's that multiplication has to be handled differently than addition and subtraction, and backtracking has to track operator precedence in real time. One wrong move and your solution either times out or returns garbage. StealthCoder makes this invisible during the live OA if the precedence trick doesn't click.
Companies that ask "Expression Add Operators"
Expression Add Operators 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. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThis is a backtracking + math problem disguised as string manipulation. Most candidates start by building expression strings and eval-ing them, which works for small inputs but explodes on larger ones. The real pattern: you can't just track a running sum. When you hit a multiply operator, you have to undo the last operand, multiply it, and re-add it. So you track the previous operand separately and update it based on the operator you just inserted. Each recursive call tries all three operators at each position, pruning branches that already exceed the target (if you're doing pruning). Common pitfall: forgetting that multiplication binds tighter, so '2+3*4' must equal 14, not 20. If precedence handling breaks, your backtracking never converges. StealthCoder surfaces a working implementation instantly if you hit this live and the operator-precedence logic slips your mind.
Pattern tags
You know the problem.
Make sure you actually pass it.
Expression Add Operators recycles across companies for a reason. It's hard-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. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Expression Add Operators interview FAQ
How hard is this really compared to other hard problems?+
It has a 42% acceptance rate, which is middle-of-the-road for hard. The conceptual trick (tracking the previous operand to handle precedence) is doable once you see it, but the implementation is verbose and error-prone under time pressure. Most flubs happen in edge cases around negative numbers and the operand undo logic.
Do Google and Meta still ask this as often?+
Yes. It's in the active rotation at both companies, along with Pinterest and LinkedIn. It's a classic hard-tier string + backtracking hybrid, so it stays relevant for roles that care about algorithm depth and operator precedence understanding.
What's the trick I'm missing if my solution times out?+
You're likely building and evaluating full expression strings at each step, or not pruning branches early. The winning approach tracks the running sum and the previous operand separately, so you only do math once per recursive call. Prune if your current value plus the best-case remaining digits still can't reach the target.
How does this relate to the Backtracking topic?+
Backtracking here means you try each of the three operators at each position, then undo and try the next one. You're building a decision tree and cutting branches that can't yield the target. The constraint is the target value itself, which shapes which branches are worth exploring.
Will I hit negative number or leading-zero edge cases?+
Almost certainly. The problem allows negative intermediate results from subtraction, and you can't have leading zeros in a multi-digit operand. Many solutions break when they parse '01' or handle something like '1-2*3' incorrectly. Test both before your OA.
Want the actual problem statement? View "Expression Add Operators" on LeetCode →