Leetcode 1429. First Unique Number

Design a data structure that, given an initial list and a stream of added numbers, returns the first integer that appears exactly once at any time. The core challenge is maintaining element frequencies while preserving insertion order so queries for the current first unique remain efficient.

Asked at:

Uber

Question Timeline

See when this question was last asked and where, including any notes left by other candidates.

Company

Level

Region

Mid February, 2026

Uber

Staff

Early February, 2026

Uber

Mid-level

Got this question on the Depth in Specialization interview. It was phrased as a system to keep track of recurrent visitors vs one time visitors. Should implement two methods: one to post a visit and other to get the first one time visitor.

Late January, 2026

Uber

Staff

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