Graph

LeetCode 1306 note: indices are nodes and the two jumps are edges; BFS or DFS decides whether a zero is reachable

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

LeetCode 3613 note: add edges from light to heavy; the weight when exactly k components remain is the answer, and 0 when n <= k

LeetCode 3608 note: add edges back from the largest time; the time of the first edge that drops the component count below k is the answer

LeetCode 3607 note: Union-Find groups, one min-heap per group, and lazy deletion for offline stations

LeetCode 3493 note: convert each row to a set, compare intersection sizes pairwise to add edges, then count components

LeetCode 1791 note: the center lies on every edge, so the common endpoint of the first two edges is the answer

LeetCode 133 note: during DFS map each old node to its copy and return the existing copy when revisited, avoiding infinite recursion; an empty graph returns None

LeetCode 997 note: the judge is trusted by the other n - 1 people and trusts nobody, so in-degree minus out-degree equals n - 1