I knew 2048 as a game but had never thought about coding the move logic.
Start by clarifying the rules and edge cases, then propose a clean algorithm that processes each row/column in the direction of movement, compacts non-zero tiles, merges adjacent equal tiles (tracking merged flags), and compacts again. Implement a helper that operates on a single line and apply it to all lines (rows or columns) with appropriate direction handling.
Pro tip: Mention that you can avoid duplicating logic for all four directions by extracting a line-processing function and using transformations (reverse, transpose) to map any direction to a single canonical left-shift operation. This demonstrates code reuse and reduces bugs.
Confirm the rules: each tile merges at most once per move, merges happen in the direction of movement, and zeros represent empty cells. Discuss edge cases like full board, no possible moves, and multiple merges in a line.
Create a function that processes a single line (array of 4 values) for a left shift: remove zeros, merge adjacent equal values (skipping the next element after a merge), then pad with zeros. This ensures each tile merges only once.
For each direction, extract lines (rows for left/right, columns for up/down), apply the line function (reversing the line for right/down), and write back. Alternatively, use matrix transformations (transpose, reverse) to reuse the left-shift logic.
Write clean code with helper functions, then test with representative cases: simple shifts, merges, multiple merges in one line, and no-op moves. Verify that the merge-once rule holds.
State that the solution is O(n^2) for an n x n board (here n=4), which is optimal since every cell must be examined. Discuss trade-offs between in-place modification and creating a new board, and between code duplication and abstraction.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.