Find the Lone Number

L2 Easy Arrays & Hashing

Concept

Frequency analysis makes "which one is odd one out?" mechanical: find the entry whose count looks different from every other.

Every integer in nums appears exactly twice, except for one integer that appears only once. Find and return that unique integer.

Examples

▸ nums = [2, 2, 3, 3, 4]

→ 4

▸ nums = [1]

→ 1

▸ nums = [5, 7, 5, 9, 7]

→ 9

Progressive Hints

Hint 1 · Nudge

Everyone has a matching partner except one. Find the odd one out by tallying the pairs.

Hint 2 · Plan

Tally how often each value appears, then return the key whose tally is exactly 1.

Hint 3 · Approach

counts = frequency map. For each key: if counts[key] == 1 return that key.

Output

// Run your code to see the output here.