First Repeated Value

L2 Easy Arrays & Hashing

Concept

Returning the *first* repeat needs an ordering trick: as soon as a value is seen twice, that is your answer.

Return the first value in nums that appears more than once. If every value is unique, return nil.

Examples

▸ nums = [2, 1, 3, 2]

→ 2

▸ nums = [1, 2, 3]

→ nil

▸ nums = []

→ nil

Progressive Hints

Hint 1 · Nudge

The first repeat is the one you bump into before any other repeat.

Hint 2 · Plan

Scan left to right, adding each value to a set. As soon as the current value is already in the set, return it. If the scan finishes cleanly, return nil.

Hint 3 · Approach

seen = empty set. For each n: if n is in seen return n, otherwise add n. Return nil.

Output

// Run your code to see the output here.