Medium

LeetCode 1448 note: pass the path maximum down during DFS; a node is good when its value is at least that maximum

LeetCode 143 note: find the middle, reverse the second half, and merge the two alternately, cutting the first half’s tail

LeetCode 138 note: the first pass maps old nodes to new ones, the second wires next and random; seed the dictionary with None -> None

LeetCode 105 note: the first pre-order value is the root, and its position in the in-order tells the left subtree’s size; look positions up in a dictionary for O(n)

LeetCode 102 note: BFS one whole level at a time, building the next level from the current one; an empty tree returns an empty list

LeetCode 981 note: store a (timestamp, value) list per key, with timestamps increasing; get binary searches the last timestamp <= the query

LeetCode 875 note: binary search the speed for the smallest k with total hours <= h, continuing left even when the hours equal h

LeetCode 424 note: a variable sliding window where window length minus the top character count is the number of replacements; shrink when it exceeds k

LeetCode 153 note: compare mid with the right end: nums[mid] > nums[r] puts the minimum to the right, otherwise mid may be it, so r = mid

LeetCode 347 note: count first, then take the k most frequent with a size-k heap or a bucket sort indexed by count

LeetCode 238 note: each answer is the product of everything to the left times everything to the right; two passes, no division

LeetCode 167 note: sorted, so move two pointers inward, shifting right when the sum is too large and left when too small; indices are 1-based