Arista Networks·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Arista Networks coding round, one algorithm question that looks simple but has a sneaky twist. Worth reviewing the classic missing number problem before you go in, because they specifically push you past the easy version.

Questions Asked (1)

Given a sorted array that contains all integers in some range [L, R] except one, find the missing number. Be prepared to discuss multiple approaches and their tradeoffs.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I jumped straight to Gauss summation and got it working, sum of L to R minus the actual array sum gives you the answer, O(n) time O(1) space.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints (e.g., array size, range, duplicates, integer overflow) and then present multiple approaches: brute force, mathematical sum, binary search, and XOR. Compare their time/space complexities and discuss trade-offs, ultimately recommending the optimal solution based on constraints.

Pro tip: Mention that binary search can be adapted to find the missing number in O(log n) time by comparing the middle element's value to its expected value, and highlight that this approach avoids integer overflow issues present in the sum method.

1. Clarify constraints and assumptions

Ask about the array size, range [L, R], whether duplicates exist, and if the array is sorted in ascending order. Confirm that exactly one number is missing and all others appear once.

2. Outline brute force and mathematical approaches

Describe a linear scan to find the first gap (O(n)) and the sum formula (expected sum minus actual sum) which is O(n) time and O(1) space but may overflow for large ranges.

3. Present optimal binary search solution

Explain that since the array is sorted, we can binary search for the missing number by checking if the middle element equals its expected value (L + index). If it does, the missing number is to the right; otherwise, it's to the left.

4. Discuss XOR approach and trade-offs

Mention that XOR of all expected numbers and array elements gives the missing number in O(n) time and O(1) space without overflow, but it's not as efficient as binary search for large n.

5. Compare and recommend

Summarize time/space complexities: brute force O(n), sum O(n) with overflow risk, XOR O(n) no overflow, binary search O(log n). Recommend binary search as optimal for sorted arrays, but note that if the array is unsorted, XOR or sum is better.

Key Points to Mention

Time and space complexity of each approach (brute force, sum, XOR, binary search).
Integer overflow risk in the sum method and how XOR avoids it.
Binary search logic: compare arr[mid] to L + mid to decide search direction.
Edge cases: missing number at the beginning or end of the range.
Assumption that the array is sorted and contains distinct integers.
Trade-offs: binary search is fastest but requires sorted input; XOR is linear but works on unsorted arrays.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.