Have I Seen This Before?

The simplest way to spot duplicates. A quick puzzle to sharpen your brain.

Puzzle

Given an integer array, return true if any value appears at least twice, otherwise return false.

Example

[1, 2, 3, 1] → true

[1, 2, 3, 4] → false

Hints (use only if stuck):

• Hint 1: You care about seen vs unseen values.

• Hint 2: Aim for O(n) time, not nested loops.

• Hint 3: Ask yourself what data structure naturally answers “have I seen this before?”

Solve it cleanly. Small, expressive code.

Consistency unlocks harder puzzles.