Pattern Recognition Drill
Medium Backtracking
The Problem
Given digits 2-9, return all possible letter combinations (phone keypad).
What approach would you use?
Think about it before scrolling down.
For each digit, try each letter. Recurse on remaining digits. Collect at end. O(4^n).
Common Trap
This is a Cartesian product, not permutations. The order of digits is fixed.