Given phone number digits, list all possible letter combinations from the keypad (like T9).
Generate every possible ordering of a list of distinct numbers.
List all ways to choose k numbers from 1 to n (order does not matter).
Find all ways to pick numbers from a list (reuse allowed) that add up to a target sum.
Place 4 queens on a 4x4 chessboard so none can attack another (no shared row, column, or diagonal).
Check if you can spell a word by tracing a path through adjacent cells in a grid (each cell used once).
Split a string into pieces where every piece reads the same forwards and backwards.
Generate every possible way to arrange n pairs of parentheses so they're all properly matched.