Hash-Table

LeetCode 763 note: record each letter’s last index, extend the current part’s end while scanning, and cut when the scan reaches it

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

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

LeetCode 146 note: a hash map finds nodes and a doubly linked list keeps the usage order; both get and updating an existing key must move the node to the front

LeetCode 889 note: keep the current path on a stack; when its top equals the next postorder value that subtree is complete and is popped; new nodes go left first

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

LeetCode 652 note: encode every subtree in post-order and report it on its second occurrence; string keys are O(n^2) worst case, (val, left id, right id) ids make it O(n)

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

Simulation by building a list, parsing by trying every pattern at each position, all substrings of length k, counting by character class, checking balanced substrings from a fixed start, and splitting into words

Counting and frequency of frequencies, using the array itself as a hash table (sign marking), counting several keys at once (rows, columns, diagonals), incremental counts from a fixed left end, and grouping by a normalized key

LeetCode 305 note: each new land cell adds an island, then merges with neighboring land, subtracting 1 per successful merge; skip repeated positions

LeetCode 2584 note: coprime products ⇔ no prime factor in both parts; record each prime’s last index, extend reach left to right, and split after i when reach == i