Hash-Table

LeetCode 3843 note: count each value, then count how many values share each count; the first element from the left whose count is its own is the answer

LeetCode 3842 note: toggle membership in a set, or keep bulbs appearing an odd number of times, then sort

LeetCode 3839 note: words sharing the first k characters are connected, so count by word[:k]; exclude words shorter than k

LeetCode 3719 note: n is only 1500, so fix the left end and extend right, maintaining distinct odds and evens in two sets

LeetCode 1570 note: store only non-zero (index, value) pairs, and compute the dot product by merging the two index-sorted lists with two pointers, multiplying only on equal indices

LeetCode 244 note: precompute each word’s positions (sorted by construction), then answer each query by merging the two position lists with two pointers, always advancing the smaller one

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 41 note: the answer is in 1..n+1; replace impossible values with n+1 (not 0) and mark v as seen with the sign of nums[v-1]

LeetCode 3684 note: all values are positive, so deduplicate and take the k largest in descending order

LeetCode 2273 note: repeatedly deleting a word that is an anagram of the previous one keeps exactly the words that are not anagrams of the last kept word; compare sorted strings

LeetCode 2122 note: the smallest number is always in lower, so enumerating its partner fixes k; after sorting, two pointers pair each smallest unmatched number with the number 2k above it

LeetCode 923 note: sort, fix the first number, and find the other two with converging pointers; equal end values are counted with a combination, different ones by multiplying each end’s duplicate count