Hash-Table

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

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

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 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

LeetCode 49 note: use each string’s sorted form as a key; strings sharing a key form one anagram group

LeetCode 36 note: each digit may appear once per row, column and 3 × 3 box; put (digit, row), (digit, column) and (digit, box) keys into one set

LeetCode 17 note: each level handles one digit and tries its letters; an empty input returns an empty list

LeetCode 1189 note: each letter supports count // needed balloons, with l and o needed twice; take the minimum

LeetCode 1160 note: a word is formable when none of its character counts exceeds chars’; sum the lengths of such words

LeetCode 1128 note: normalize each domino to (small, large) as a key and count; a key with c dominoes yields c(c - 1) / 2 pairs

LeetCode 1122 note: count first, emit values in arr2’s order, then append the remaining values in ascending order