Medium

LeetCode 1186 note: Kadane with one more state, ending here with no deletion or with one deletion; update the answer at every step

LeetCode 775 note: turn a counting problem into an existence check and solve it in O(n)

LeetCode 912 note: hand-written merge sort and the two bugs I hit

LeetCode 253 note: a heap of end times for running meetings; half-open intervals need <=

LeetCode 2761 note: sieve primes below n, then try x up to n / 2; when x and n - x are both prime, they form a pair

LeetCode 2523 note: sieve the primes in range, since only adjacent primes can be closest; or trial-divide rightward from left and stop at the first twin primes with gap 2

LeetCode 4067 note: subarrays of a valid window stay valid, so slide; adding x only needs checks involving x: x as an addend (x + y in the window) or x as the sum (x - y in the window)

LeetCode 4045 note: merge robots already close at t = 0, then go right to left like car fleets, where anything faster than the fleet ahead eventually joins it

LeetCode 622 note: a fixed array plus head and count, every position taken modulo the capacity, and count deciding empty versus full

LeetCode 4019 note: kept characters never change again, so record each character’s last position in the result; a new character within k of it gets merged away

LeetCode 323 note: count components with DFS or Union-Find; Union-Find without path compression is much slower

LeetCode 4040 note: with multiplications before divisions, the best is always multiply-only or divide-only; list each number’s reachable values and costs, then run a grouped 0/1 knapsack