Graph

LeetCode 834 note: rerooting DP: compute subtree sizes and ans[0] with root 0, then push ans[child] = ans[parent] - size + (n - size) to every node

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 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

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

LeetCode 3203 note: connect the two centers; the answer is max(d1, d2, ceil(d1/2) + ceil(d2/2) + 1)

LeetCode 3313 note: the last marked node is the farthest one, and a farthest node can always be a diameter endpoint

LeetCode 2608 note: BFS from every node; a visited neighbor that is not the parent closes a cycle of length dist[u] + dist[v] + 1

Ordering, cycle detection, dependency unlocking, and graph propagation

Dijkstra, minimax paths, and Bellman-Ford with edge limits

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

LeetCode 323 note: count components with DFS or Union-Find; Union-Find without path compression is much slower