Number of Digit One
A hard-tier problem at 36% community acceptance, tagged with Math, Dynamic Programming, Recursion. Reported in interviews at Google and 1 others.
Number of Digit One is a hard problem that Google and Atlassian ask, and it's a genuine wall-hitter for unprepared candidates. You're given a number and must count how many times the digit 1 appears in all integers from 0 to that number. The naive approach of iterating and counting fails instantly on large inputs. This is a Math and Dynamic Programming problem that rewards pattern recognition over brute force. If this hits your live assessment and you blank on the counting logic, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Number of Digit One"
Number of Digit One 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 recognizing that you can't iterate. Instead, count digit 1s by position (ones place, tens place, hundreds place, etc.) and derive a closed-form calculation for each. For each digit position, the answer depends on the higher digits, current digit, and lower digits of the number. The pattern feels abstract until you work through a small example by hand, then the recursion or DP solution clicks. Common pitfalls: overthinking the formula, off-by-one errors in range calculations, and failing to handle edge cases where the current digit is 0, 1, or greater than 1. Most candidates who see this without preparation either timeout or produce incorrect counts. This problem sits at the intersection of combinatorial counting and digit DP, making it rare enough that drilling standard array/string problems won't help. StealthCoder is the hedge for the moment you hit this and realize iteration won't scale.
Pattern tags
You know the problem.
Make sure you actually pass it.
Number of Digit One 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. 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.
Number of Digit One interview FAQ
Is this really asked at Google and Atlassian, or is it just a LeetCode hard?+
Yes, both companies report asking it. It's not common, but it appears in their real assessment pipelines. The acceptance rate is 36 percent, which means most candidates either timeout or get wrong answers on first attempt. It's worth knowing the pattern.
What's the actual trick that makes it not brute-force?+
You count 1s by digit position rather than iterating through all numbers. For each place value (ones, tens, hundreds), calculate how many 1s appear there based on the digits above and below it. This reduces O(n) to O(log n). The formula depends on whether the current digit is 0, 1, or greater than 1.
Do I need to memorize a specific formula, or is there a general approach?+
There's no formula to memorize. Instead, learn the digit-by-digit decomposition method and practice deriving the count for one position at a time. Once you understand why the current digit matters, the recursion or DP solution follows naturally. Most solutions use a helper function that processes each digit position.
How does this relate to the Math and Dynamic Programming topics?+
The Math part is the digit-position analysis and combinatorial counting. The DP part emerges if you memoize the digit extraction and counting logic to avoid recalculating the same subproblems. Some solutions lean more mathematical, others use memoized recursion. Both are valid.
If I haven't seen this pattern before, how deep do I need to drill to be ready?+
One solid pass through the logic, working through a number like 234 by hand, should do it. The pattern isn't intuitive, so drilling standard problems won't help. If you haven't touched this specific problem type, it's a knowledge gap worth filling before an OA with Google or Atlassian.
Want the actual problem statement? View "Number of Digit One" on LeetCode →