Hard

LeetCode 32 note: a stack of indices whose bottom is the position just before the current valid run; the length is i - stack[-1]

LeetCode 25 note: count the length to know how many full groups to reverse, k nodes each; a dummy head avoids special-casing the first group, and the old group head becomes its tail

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 460 note: key → node, freq → doubly linked list, plus a min_freq; within the same frequency, LRU decides what to evict

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 968 note: post-order greedy with three states per node: has a camera, covered, not covered; place a camera only when a child is uncovered, and add one if the root is left uncovered

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

LeetCode 124 note: DFS returns the best downward chain and updates the answer with left chain + node + right chain at every node; drop negative chains

LeetCode 23 note: a heap that holds only k heads, and divide-and-conquer pairwise merging

LeetCode 493 note: when the counting condition differs from the sort order, scan first, then merge

LeetCode 4051 note: prefix sums, complement counting, range counting, and the k = 0 edge case

LeetCode 2426 note: simplify the inequality, then decide whether the matches form a prefix or a suffix