Matrix

Transposing and rotating, converting between 2D indices and a 1D sequence, neighbors and diagonals, and spiral traversal with four boundaries

Connected components, grid flood fill, multi-source BFS, state-space BFS, shortest cycles, and recursion depth in DFS

LeetCode 827 note: label every island and record its size, then for each 0 add the sizes of the distinct neighboring islands plus 1

LeetCode 212 note: build a Trie of all words, walk it during the grid DFS, remove words once found, and prune empty nodes

LeetCode 2267 note: the state is (r, c, open balance); the balance can be neither negative nor more than the remaining steps; search the states with BFS / DP and succeed at the end with balance 0

LeetCode 1210 note: the BFS state is (tail position, orientation), each move checks its own empty cells, and equal step costs need no heap

LeetCode 4046 note: Dijkstra with the last direction and the turns used in the state; for the same cell and direction, an earlier pop is cheaper, and if it also used fewer turns it dominates later states

LeetCode 773 note: there are only 6! = 720 boards, so flatten each board into a string as a BFS state, with swaps of 0 and a neighbor as the moves

LeetCode 499 note: Dijkstra prioritized by (distance, path string), and the ball stops when it rolls over the hole

LeetCode 864 note: the BFS state is (position, bitmask of keys), so the same cell with different keys is a different node

LeetCode 2328 note: strictly increasing means no cycles; dp[cell] = 1 + Σ dp[larger neighbors], processed from largest to smallest value (or by memoized DFS), summed over all cells

LeetCode 1292 note: 2D prefix sums plus binary search on the side length; never mix the two binary search styles