Hard

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 1028 note: a stack holds the path from the root to the current node, so its length is the depth

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 2509 note: in heap numbering the parent is v // 2, so climb the larger node until both meet at the LCA

LeetCode 301 note: compute the minimum left and right deletions before backtracking, or BFS one deletion per level and stop at the first valid level

LeetCode 1425 note: dp[i] = nums[i] + max(0, max of the previous k dp values); that max is a sliding window maximum, available in O(1) from a monotonic deque

LeetCode 1373 note: a post-order traversal returns (sum, min, max, is_bst); a node roots a BST when the left max < its value < the right min

LeetCode 996 note: with n at most 12, use bitmask DP: dp[mask][last] = permutations using the set mask and ending with last; dedupe by using equal values in index order

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 1354 note: work backwards, since the maximum was set last; jump with a modulo when the gap is large

LeetCode 719 note: binary search on the distance, counting pairs with distance <= mid using a sliding window

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