#matrices

20 matrices interview questions, asked at Meta, Google, Amazon and 8 more companies. Last asked Sep 2026.

Rank transform of a matrix with distinct valuesCodingGiven an m x n matrix of distinct integers, replace each value with a rank starting at 1 so that in any row or column a larger value gets a larger rank, and ranks are as small as…
Last asked Sep 2026
+1
Sparse matrix multiplicationCodingMultiply two matrices that are mostly zeros. Store only the non-zero entries and skip work on zeros, and state the cost in terms of the number of non-zero entries.
Last asked Sep 2026
2D max pooling that also returns where each maximum came fromCodingImplement 2D max pooling over a grid of integers: a k by k window moves across the grid with a given stride, and each window outputs its maximum. Follow-up: for every output cell…
Last asked Sep 2026
Copy a rectangle within a 2D array in placeCodingGiven a 2D array, a rectangular block (top-left corner, height, width) and a destination for its top-left corner, copy the block to the destination with O(1) extra memory. The…
Last asked Sep 2026
Tic-tac-toe with K playersCodingImplement tic-tac-toe for k players on an n x n board. Players take turns placing their marks. However large the board is, a player wins as soon as they have three of their marks…
Last asked Jul 2026
Leftmost column with a one in a row-sorted binary matrixCodingIn an m x n matrix of 0s and 1s, each row has all its 0s before its 1s. Return the index of the leftmost column that holds a 1, or -1 if there is none. Start with a binary search…
Last asked May 2026
Multiply two sparse matricesCodingMultiply two matrices that are mostly zeros. Skip the zero entries: store only the non-zero values of each matrix (by row or as lists of positions) and multiply only those pairs…
Last asked May 2026
Game of Life, updated in placeCodingEach cell of a grid is alive or dead, and all cells update together from the states of their eight neighbors, as in Conway's Game of Life. Compute the next state in place, without…
Last asked May 2026
Rotate a matrix by 90 degreesCodingRotate a square matrix 90 degrees clockwise, in place.
Last asked Apr 2026
Longest increasing path in a matrixCodingReturn the length of the longest path in an integer matrix that moves up, down, left or right and only to strictly larger values. Use memoised DFS (or a topological order) so…
Last asked Apr 2026
+1
Spiral matrixCodingReturn all elements of an m x n matrix in spiral order: along the top row, down the right column, back along the bottom row, up the left column, then inward.
Last asked Apr 2026
Rotate an image matrix 90 degrees in placeCodingRotate a square matrix by 90 degrees clockwise without allocating another matrix. Transpose it and then reverse each row, or rotate the four cells of each ring in place. Explain…
Last asked Apr 2026
Update a grid of firing neuronsCodingA grid holds neurons that are either firing or not. In each step every neuron's next state depends on how many of its neighbours are firing, in the style of the Game of Life…
Last asked Feb 2026
Diagonal traversal of a matrixCodingReturn the elements of a matrix diagonal by diagonal. In one version the direction alternates (up-right, then down-left); in another every diagonal is read in the same direction…
Last asked Feb 2026
Matrix transforms and what eigenvalues meanCodingImplement basic matrix operations used to transform points (multiply, rotate, scale, compose transforms). Then explain eigenvalues and eigenvectors: what they say about a…
Last asked Feb 2026
Rotate a 3x3 grid of digits by 180 degreesCodingRotate a 3x3 grid of digits by 180 degrees and return the result, in place if you can. Only the title of this report was public.
Last asked Jan 2026
Validate a tic-tac-toe board stateCodingGiven a tic-tac-toe board in the middle or at the end of a game, decide whether it could have been reached by legal play: X moves first, players alternate, and nobody moves after…
Last asked Dec 2025
Kth smallest element in a sorted matrixCodingEvery row and every column of an n x n matrix is sorted ascending. Return the k-th smallest value. Compare a min-heap over the rows with binary search on the value range.
Last asked Sep 2025
Valid Sudoku in one passCodingDecide whether a partly filled 9x9 Sudoku board is valid: no digit repeats in any row, column or 3x3 box (empty cells are ignored). Check it in a single pass over the board.
Last asked Aug 2025
Toeplitz matrixCodingReturn whether every diagonal of a matrix running from top-left to bottom-right holds a single repeated value, comparing each cell with the one up and to its left.
Last asked Aug 2025