Medium

LeetCode 148 note: split in half with fast and slow pointers, sort each half recursively and merge; O(n log n) with no extra array

LeetCode 473 note: the total must be divisible by 4; place sticks from longest to shortest into one of four sides, skipping a side that would overflow or that equals a side already tried

LeetCode 435 note: minimum removals = total minus the maximum kept; sort by right end and greedily keep the one ending earliest

LeetCode 397 note: even numbers are always halved; for odd numbers try +1 and -1 with memoized recursion, since n is too large for an n-sized array

LeetCode 396 note: F(k) and F(k - 1) differ by one formula: F(k) = F(k - 1) + sum - n × the element moved to the front

LeetCode 337 note: tree DP where each node returns the best (rob it, skip it) pair; not robbing alternate levels

LeetCode 313 note: generalizing LC 264 with one pointer per prime; with a heap, avoid pushing the same number many times

LeetCode 50 note: fast exponentiation, x^n = (x²)^(n/2) with an extra x for odd n; for negative n compute 1 / x^(-n)

LeetCode 264 note: three pointers mark the next ugly numbers to multiply by 2, 3 and 5; take the minimum each time and advance every pointer that produced it

LeetCode 187 note: count every length-10 substring in a hash map and return those seen more than once

LeetCode 142 note: after fast and slow meet, send one back to head and step both one at a time; they meet again at the cycle entry

LeetCode 227 note: read whole numbers, and at each operator apply the previous one: + and - push the number (negated for -), * and / combine with the top