Medium

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 452 note: sort by end and shoot at the end; only a balloon starting after the arrow needs a new one, and touching ends count as overlapping

LeetCode 1011 note: binary search on the answer over [max(weights), sum(weights)], checking the day count by filling greedily in order

LeetCode 4099 note: the swaps needed equal the inversion count; every locked position some element must cross has to be unlocked, so count those, compare with k, and add the inversions counted with merge sort

LeetCode 4098 note: value’s smallest prime factor p decides everything: the next element is at least value + p, then even numbers step by 2; the left side is feasible iff value - p >= 2 * index

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

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 437 note: prefix sums on a tree with a hash map: look up prefix - target before adding the current prefix, and undo it when leaving the node

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)