First ticker that trades exactly once

Question

You receive the tape as an ordered list of traded symbols. Return the first symbol whose total count in the whole tape is exactly one. If every symbol repeats, return None.

tape = ["AAPL", "MSFT", "AAPL", "GOOG"]
-> "MSFT"       # AAPL repeats; MSFT is the first that never does

Return the first symbol with total frequency one, in tape order. Aim for $O(n)$ .

Accepted Answer

First pass: count every symbol. Second pass: walk the tape again in order and return the first symbol whose count is one. Two passes keep the code simple and preserve first-seen order, which a single-pass approach would have to track separately. Tracing ["AAPL", "MSFT", "AAPL", "GOOG"]: counts are AAPL: 2, MSFT: 1, GOOG: 1. The first symbol in order with count one is MSFT. Time O(n), space O(u) for the u distinct symbols. Edge cases: an empty tape returns None, and a tape where everything repeats also returns None. Takeaway. When a question mixes "first in order" with "counted globally," a co…

First ticker that trades exactly once

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