Counting

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

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

LeetCode 1782 note: count pairs with deg[a] + deg[b] > q using sorted degrees and converging pointers, then correct the over-count for pairs that share edges

LeetCode 992 note: “exactly k kinds” = “at most k” − “at most k - 1”, where each at-most window adds right - left + 1 per right end

LeetCode 3805 note: shift each string so its first letter is a, making Caesar-equivalent strings identical, then count pairs per normalized form

LeetCode 3679 note: the rule already decides what must be discarded, so simulate it; the length-w window counts only kept arrivals, and discarded ones must not be subtracted when they leave

LeetCode 1512 note: count while scanning; each number pairs with every earlier equal number, so add its current count to the answer

LeetCode 3712 note: count each value, and for counts divisible by k add value × count

LeetCode 2131 note: pair each word with its reverse on both sides; self-palindromes also pair with each other, and only one leftover can sit in the middle

Remainder pairing with a hash map; handle 0 and k/2 remainders carefully.

LeetCode 3639 note: each new star adds (left distance) × (right distance) new substrings; or binary search on time

LeetCode 3545 note: reducing the number of kinds means deleting whole kinds, starting from the least frequent