Matrix

LeetCode 3643 note: flip only the submatrix rows, swapping the slices of rows x + i and x + k - 1 - i

LeetCode 1222 note: step outward from the king in eight directions; the first queen met in each direction can attack, with queens in a set for lookups

LeetCode 3619 note: flood fill each island and sum its values; with up to 10^5 cells, iterate to avoid deep recursion

LeetCode 3071 note: count each value on and off the Y, enumerate the Y value a and background value b (a != b), and take the fewest cells to change

LeetCode 3070 note: submatrices containing the top-left corner are exactly the 2D prefix sums P[i][j]; count those <= k

LeetCode 240 note: start at the top-right corner, moving left when larger and down when smaller, eliminating a row or column each step

LeetCode 48 note: rotating 90 degrees clockwise is a transpose followed by reversing each row, both in place

LeetCode 417 note: walk inward from each ocean’s shore to cells no lower than the current one; cells reached from both oceans are the answer

LeetCode 74 note: each row continues the previous one in sorted order, so binary search it as a 1-D array of length m × n, mapping mid to (mid // n, mid % n)

LeetCode 73 note: record which rows and columns contain a 0 first, then update afterward; the follow-up uses the first row and column as markers

LeetCode 59 note: keep four boundaries as in LC 54, but fill in 1 to n² instead of reading

LeetCode 54 note: keep four boundaries and walk right, down, left and up, shrinking a boundary after each side