Hard

LeetCode Problem Solving

LeetCode 2402 note: one heap for free rooms and one for busy rooms; a delayed meeting keeps its original length

LeetCode 927 note: every part has the same number of 1s, so count them; the third part always runs to the end of the array, fixing what each part must match, then check the parts do not overlap

LeetCode 952 note: sharing a factor ⇔ sharing a prime factor; factor each number and union indices with the same prime factor using Union-Find, with no pairwise GCDs

LeetCode 3518 note: a palindrome is determined by its left half, so this is the k-th permutation of a multiset; try characters position by position and use permutation counts, updated in O(1), to find k’s branch

LeetCode Problem Solving

LeetCode 154 note: compare with the right end as in LC 153; on equality the break could be on either side, so r -= 1, worst case O(n)

LeetCode 42 note: each cell holds min(tallest on the left, tallest on the right) - its own height; use prefix and suffix maxima or two pointers