Breadth-First-Search

LeetCode 2385 note: turn the tree into an undirected graph and BFS from start; the number of levels minus one is the answer

LeetCode 2858 note: rerooting DP: count the reversals needed with root 0, then moving the root to a child adds 1 if that edge pointed down and subtracts 1 if it pointed up

LeetCode 863 note: turn the tree into an undirected graph (adding edges to parents), then BFS k steps from target

LeetCode 449 note: for a BST, pre-order without null markers is enough; deserialize recursively with bounds in O(n)

LeetCode 297 note: pre-order with null markers identifies the tree uniquely; deserialize by consuming tokens from an iterator

LeetCode 515 note: BFS level order, taking the maximum of each level; start from -inf

LeetCode 429 note: BFS level order with the children list in place of left and right

LeetCode 3787 note: two BFS passes find one diameter’s ends A and B; every node’s farthest node is A or B, so a node is a diameter endpoint exactly when max(dist(A,i), dist(B,i)) equals the diameter

LeetCode 314 note: BFS with a column index; BFS is what guarantees top-to-bottom within a column and left-to-right within a row

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 4041 note: with operations in any order, BFS finds every value a number can become (up to sum) with the fewest steps, then the same grouped 0/1 knapsack as 4040

LeetCode 488 note: with at most 16 balls on the board and 5 in hand, memoize DFS on the whole (board, hand) state, inserting only before a same-colored ball or between two equal balls