Matrix

LeetCode 36 note: each digit may appear once per row, column and 3 × 3 box; put (digit, row), (digit, column) and (digit, box) keys into one set

LeetCode 892 note: a stack of v cubes has 4v + 2 faces; subtract 2 × min for each overlap with the stacks above and to the left

LeetCode 883 note: the top view counts nonzero cells, and the front and side views sum each row’s and each column’s maximum

LeetCode 867 note: the transpose is n × m with res[j][i] = matrix[i][j]; in Python, zip(*matrix)

LeetCode 832 note: reverse each row and invert its bits, which two pointers can do at once

LeetCode 766 note: every diagonal is constant exactly when each cell equals its upper-left neighbor

LeetCode 733 note: DFS or BFS from the start, recoloring 4-directional cells that share the start’s original color; return immediately if the colors are equal

LeetCode 661 note: each cell becomes the floored average of the existing cells in its 3 × 3 neighborhood, written into a new matrix

LeetCode 566 note: return the original if the element counts differ; otherwise element k goes to (k // c, k % c)

LeetCode 463 note: each land cell contributes 4 edges, minus 2 for each land neighbor above or to the left