Medium

LeetCode 777 note: XL→LX moves L left and RX→XR moves R right; after skipping X the order must match, with L only moving left and R only moving right

LeetCode 3829 note: one queue each for riders and drivers; cancellations only update a set, and canceled riders at the front are dropped before matching

LeetCode 3828 note: Alice can leave only the first or last element on her first move, guaranteeing max(first, last); Bob can likewise keep only an end, and one deletion changes at most one end, so the answer is max(first, last)

LeetCode 2337 note: after skipping blanks the piece order must match; L only moves left and R only moves right, so each L’s start index must be >= its target index and each R’s must be <=

LeetCode 809 note: two pointers compare runs of equal characters; when the lengths differ, the run in s must be at least 3 and no shorter than the word’s run, and both strings must end together

LeetCode 442 note: values are in 1..n, so mark v with the sign of nums[v-1]; on the second visit it is already negative

LeetCode 3824 note: binary search on k with a monotonic check; the upper bound must reach n because there are at least n operations

LeetCode 581 note: scanning left to right with a running max, the last index below it is the right boundary; scanning right to left with a running min, the last index above it is the left boundary

LeetCode 3555 note: every length-k window is one instance of LeetCode 581; with n only 1000, two O(k) scans per window are enough

LeetCode 3323 note: merge first; a new interval can join groups l through r ⇔ start of group r − end of group l <= k, so a sliding window finds the most groups one interval can connect

LeetCode 1989 note: process the “it” players left to right, each catching the leftmost uncaught person in range, with a right-only pointer to the next candidate

LeetCode 658 note: shrink two pointers from both ends to length k, or binary search the window start