🧠 Drill · Think it through
The unpaired locker
The problem
A wall of lockers is labeled with numbers. Every number appears exactly twice except for a single number, which appears once. Return that lone number. You may use only constant extra memory, and each pair of equal numbers should cancel out.
Don't jump to code. Walk the four questions an expert asks first — you'll build the habit, not just the answer.
1
Restate it
Before anything else — did you read what's actually being asked? Strip the story to the bare goal.
Strip away the story. What are we actually asked to return?