# Start with seen = set()

Canonical URL: <https://datadriven.io/problems/start-with-seen-set-given-visitors-a-b-a-c-69f74454>

Domain: Python · Difficulty: medium

## Problem

Start with seen = set(). Given visitors = ['a', 'b', 'a', 'c', 'b', 'd'], add each visitor to seen using a loop. Print the length of seen.

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