LeetCode

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

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