Coding

Reconstruct the missing numbers

Asked at

Arista Networks

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

Stuck? Show a way to structure it+

01Confirm range and output
02State a set baseline
03Choose sign marking or cyclic placement if valid
04Restore or avoid mutation
05Cover boundaries

Reference answer

Define the range before reconstructing

Ask whether values should cover 1..n, 0..n, an arbitrary interval, and whether one or many values may be missing. A hash-set baseline is clear: insert observed values, then scan the expected range and emit absent values. It uses O(n + r) time and O(n) space for range size r. If values are guaranteed in 1..n and mutation is allowed, mark abs(value)-1 negative or cyclically place each value; the unmarked or misplaced indexes are missing, with O(n) time and O(1) auxiliary space. Explain that duplicates and out-of-range values need an explicit policy. Test missing endpoints, no missing values, repeated values, one element, and inputs where marking would overflow or violate immutability.

Then expect these follow-ups

When is sign marking invalid?
Tests: follow-up reasoning
How would you return both missing and duplicate values?
Tests: implementation extension

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