Linked-List

LeetCode 328 note: build one list from odd positions and one from even positions, then append the even list to the odd one; remember to cut the even list’s tail

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 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

Dummy heads, reversal (whole list, a range, half), fast and slow pointers for middles and cycles, two pointers n apart, pairwise swaps, two lists meeting, and copying a list with random pointers via a hash map

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

LeetCode 21 note: the dummy node, and why Python assignment is not a copy

LeetCode 1290 note: walk head to tail, shifting the running value left by one and adding the new bit

LeetCode 2487 note: the kept nodes are non-increasing left to right; keep a monotonic stack, pop anything smaller than the new node, then link what remains

LeetCode 92 note: walk a dummy to the node before left, then move the following nodes one at a time to just after it, right - left times

LeetCode 109 note: take the middle found with fast and slow pointers as the root and recurse on both halves; the height balances naturally

LeetCode 117 note: connect next pointers level by level with BFS; for a non-perfect tree, looking only at the neighbour’s children breaks the chain