Medium

LeetCode 128 note: put everything in a set and count upward only from values whose x - 1 is absent; each value is seen at most twice, O(n)

LeetCode 90 note: sort first; for a number equal to the previous one, extend only the subsets created in the previous step

LeetCode 86 note: two dummies collect nodes below x and at least x, then join them, setting the second part’s tail to None

LeetCode 82 note: every duplicated value goes, the head included, so use a dummy; on a duplicate skip to the next different value and link prev.next past it

LeetCode 81 note: with duplicates, nums[l] == nums[mid] hides which half is sorted, so just move l right by one; worst case O(n)

LeetCode 80 note: keep each value at most twice by comparing with nums[k - 2], two slots back from the write position

LeetCode 79 note: backtrack from every cell, matching the next character in four directions with visited cells marked temporarily and restored; prune early with character counts

LeetCode 78 note: each element is either taken or not; iteratively, each new number copies every existing subset with the number appended

LeetCode 77 note: choose k of 1 to n with dfs(start, path), picking the next only after start to avoid duplicate combinations

LeetCode 75 note: the Dutch flag, with 0s left of low and 2s right of high; after swapping a 2 to high, do not advance, since the swapped-in value is unchecked

LeetCode 74 note: each row continues the previous one in sorted order, so binary search it as a 1-D array of length m × n, mapping mid to (mid // n, mid % n)

LeetCode 73 note: record which rows and columns contain a 0 first, then update afterward; the follow-up uses the first row and column as markers