Sum Leftmost Subtraction Levels
Reported by candidates from ZipRecruiter's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Most people read the stopping rule too fast and subtract across the whole array, which is exactly how this ZipRecruiter question, reported in October 2023, punishes you. Sum Leftmost Subtraction Levels looks like a pile of arithmetic, but it's a plain simulation with one sharp rule: the subtraction run ends at the first value smaller than x, and a zero counts as smaller. Miss that and example 2 comes out wrong. The constraints are small, so brute force is the intended route. You have an OA coming, so spend your energy on getting the loop exactly right. StealthCoder runs invisibly on your screen as a safety net if you blank mid-assessment.
The problem
Repeat until every nonnegative value is zero: Choose the leftmost nonzero value x and add x to the result. Starting there, subtract x from consecutive values while each current value is at least x; stop before the first smaller value. Return the accumulated result. Function sumSubtractionLevels(values: int[]) → long Examples Example 1 values = [3,3,5,2,3] return = 6 The reported process accumulates six. Example 2 values = [3,3,5,2,4] return = 7 The successive added levels are 3, 2, and 2. Constraints 1 <= values.length <= 1000 0 <= values[i] <= 1000
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is that there isn't one. Simulate it. Scan for the leftmost nonzero value x, add x to a long accumulator, then walk right from that index subtracting x while the current value is at least x. Stop at the first value below x. Zeros stop the run, because zero is less than x. Repeat until the array is all zeros. Every pass zeroes at least the leftmost nonzero element, so you do at most n passes of O(n) work. That's about a million operations at n = 1000. The common pitfalls: continuing past a smaller value, subtracting from a value that is below x and going negative, and using a 32-bit int for the sum. Trace example 2 by hand first. You should see the levels 3, 2, 2. If the loop logic slips under pressure, StealthCoder is the hedge for the live OA.
The honest play: practice the pattern, and have StealthCoder ready for the one you didn't see coming.
You can drill Sum Leftmost Subtraction Levels cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built for the candidate who saw this exact problem leak two days before his OA and wondered if anyone had a play.
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Sum Leftmost Subtraction Levels FAQ
How hard is Sum Leftmost Subtraction Levels really?+
Easy to medium. The constraints (length up to 1000, values up to 1000) let a direct simulation pass. The difficulty is reading the stopping rule correctly and handling the edge cases, not finding a clever algorithm.
What's the trick to this ZipRecruiter problem?+
Follow the process literally. Find the leftmost nonzero x, add it to the result, subtract x across the consecutive run of values that are at least x, and stop at the first smaller one. Repeat until all zeros. No data structure needed.
Will brute force time out?+
Unlikely. Each pass zeroes at least the leftmost nonzero element, so there are at most n passes, each O(n). With n up to 1000 that's roughly a million steps, which is fine for any mainstream language.
What edge cases should I test?+
An array that's already all zeros, which returns 0. A zero in the middle that stops a run. Equal values in a row. A single element. Large sums, so use a 64-bit accumulator. Also trace example 2 to confirm the levels 3, 2, 2.
How do I prepare in 48 hours for this kind of OA?+
Practice writing clean simulation loops with explicit stop conditions, and hand-trace small examples before coding. Check the examples from this problem, since they show how the array changes after each pass. Then test zeros and long sums.