This one took me a while to even set up correctly.
Model the problem as a constraint satisfaction problem and use backtracking to place tiles row by row, trying all 4 rotations at each cell. At each step, only check constraints with already-placed neighbors (top and left) to prune early. If all tiles are placed successfully, return true; otherwise backtrack.
Pro tip: Precompute all 4 rotations for each tile and represent edges as integers or characters for O(1) comparisons. Also, consider sorting tiles by number of unique edges or using a frequency map to detect impossible cases early, showing you think about optimization beyond brute force.
Confirm grid dimensions R and C, tile edge labels, and that rotations are allowed. Ensure N = R * C and discuss edge cases like N=0 or impossible counts.
Decide to place tiles cell by cell in row-major order. At each cell, try each unused tile in all 4 rotations, checking only the top and left neighbors to prune invalid placements early.
Write a recursive function that takes the current position and a set of used tiles. If position exceeds grid, return true. For each candidate tile and rotation, if constraints match, mark used, recurse, and unmark on failure.
Precompute rotations for each tile. Optionally, sort tiles or use frequency counts of edge labels to fail fast if counts are odd or mismatched. Discuss time complexity O(N! * 4^N) worst-case but pruned heavily.
Walk through a small example (e.g., 2x2) to demonstrate correctness. Mention testing edge cases: no solution, multiple solutions, and performance for maximum N.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by clarifying the input format and command semantics, then propose a 2D array (or list of lists) to represent the canvas. Process commands in order, overwriting cells as needed, and finally print the canvas row by row.
Pro tip: Mention that you can optimize by storing only the final state and avoiding unnecessary redraws, but prioritize clarity and correctness first. Also, discuss how you would handle edge cases like out-of-bounds coordinates or empty commands.
Ask about the input format (e.g., command strings, function calls), coordinate system (0-indexed or 1-indexed), and behavior for out-of-bounds coordinates. Confirm that later commands overwrite earlier ones.
Use a 2D array (list of lists) of characters, initially filled with a default character (e.g., space or '.'). Ensure dimensions H x W are correctly handled.
Write separate functions for filling a rectangle, drawing a horizontal line, and drawing a vertical line. Each function should iterate over the specified range and set the character, overwriting existing values.
Iterate through the list of commands in the given order, calling the appropriate handler for each. This ensures overwriting behavior is respected.
Print each row of the canvas as a string, joining characters. Ensure the output matches the expected format (e.g., no extra spaces).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Use two passes: first compute prefix products (product of all elements before each index) and store them in the output array; then traverse from right to left, maintaining a running suffix product and multiply it into each output element. This achieves O(n) time and O(1) extra space because only the output array is used for storage.
Pro tip: Clarify that the output array is not counted as extra space, and mention that this approach avoids division, which is important when the array contains zeros. Also, discuss edge cases like empty array or single element.
Confirm that the output array is not considered extra space and discuss handling of zeros, empty arrays, and single-element arrays.
Initialize the output array and fill it such that each position i holds the product of all elements before i.
Traverse from right to left, maintaining a running suffix product, and multiply it into the output array at each index.
State that time is O(n) and extra space is O(1), then walk through a small example to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
The merge rule is the part that trips people up.
Clarify the rules: for each row (or column) in the direction of movement, extract non-zero tiles, merge adjacent equal values once (left-to-right for left/up, right-to-left for right/down), then pad with zeros. Implement a helper that processes a single line, then apply it to all lines with appropriate indexing for the given direction.
Pro tip: Mention that you'll write a helper function to process a single line, then reuse it for all rows/columns by transposing or reversing as needed—this keeps the code DRY and avoids direction-specific bugs.
Confirm that each tile can merge only once per move, and that merges happen in the direction of movement (e.g., leftmost pair first when moving left). Ask about handling empty grids or invalid directions.
Write a function that takes a list of 4 integers and returns the merged list for a left move: filter non-zeros, merge adjacent equals once, then pad with zeros.
For left/right, apply the helper to each row (reversing for right). For up/down, apply to each column (reversing for down) by transposing or using column extraction.
Code the solution, then test with examples like [2,2,2,0] left -> [4,2,0,0], and edge cases like [2,2,2,2] left -> [4,4,0,0] and [4,4,8,8] left -> [8,16,0,0].
State that time is O(n^2) for an n x n grid (here n=4), and space is O(n) for the helper. Discuss in-place vs. new grid trade-offs.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.