LeetCode

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

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 4097 note: only merge the fixed pairs (0,1), (2,3), …, stepping by 2, not every adjacent pair

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

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