Matrix

LeetCode 2392 note: row and column conditions are independent, so run one topological sort for each; put x at its row and column positions, and return an empty matrix if either has a cycle

LeetCode 3548 note: try each horizontal cut (transpose for vertical); the difference must equal a cell on the larger side, and a one-row or one-column side can only drop an end cell, while any cell works otherwise

LeetCode 980 note: grid backtracking that marks visited cells in place and checks at the end that every empty cell was covered

LeetCode 3989 note: the kept columns form a subsequence constrained only between neighbors; an LIS-style DP with dp[j] = the longest valid sequence ending at column j

LeetCode 3651 note: compute layer by layer over teleports; arriving at a cell by teleport costs the previous layer’s minimum over cells with value >= it, found with a running minimum after sorting, followed by the usual right / down grid DP

LeetCode 1878 note: enumerate every top vertex and size, walk the four edges, and keep the three largest distinct sums

LeetCode 1727 note: compute each row’s heights of consecutive ones above; columns can be reordered, so sort heights descending and the j-th tallest times j + 1 is a candidate

LeetCode 3961 note: every device but one donates its smallest unit to the same device; that device’s rating becomes the global minimum, so pick the one with the smallest second-smallest value

LeetCode 3905 note: multi-source BFS one layer at a time, collecting the maximum color per cell before writing the layer back

LeetCode 3898 note: the number of ones in row i of the adjacency matrix is vertex i’s degree

LeetCode 1975 note: minus signs can travel anywhere in pairs but the parity of the negative count never changes; with an odd count, leave one on the smallest absolute value

LeetCode 3195 note: track the topmost, bottommost, leftmost and rightmost 1; the area is height times width