Easy

LeetCode 543 note: the diameter may skip the root; return heights in post-order while updating the answer with left + right height at every node, counting edges

LeetCode 541 note: in every block of 2k reverse the first k; when fewer than k remain reverse them all, which slicing handles automatically

LeetCode 530 note: a BST’s in-order is ascending, so the minimum difference is between in-order neighbors; remember the previous value and compare

LeetCode 521 note: identical strings give -1; otherwise the longer string itself is the answer

LeetCode 520 note: only three forms are valid: all caps, all lowercase, or only the first letter capitalized

LeetCode 509 note: the starting point of 1D DP; only the previous two terms are needed, so roll two variables

LeetCode 507 note: divisors come in pairs, so enumerate up to √n and add both d and n / d for each divisor found

LeetCode 506 note: sort indices by score descending, give medals to the top three and rank strings to the rest, writing back into the original positions

LeetCode 504 note: repeatedly take % 7 and // 7 of the absolute value, then add the sign and reverse; handle 0 separately

LeetCode 501 note: a BST’s in-order is sorted, so equal values are contiguous; count the current run during in-order, resetting the answer on a new maximum and appending on a tie

LeetCode 500 note: make a set for each keyboard row and keep words whose lowercase character set is a subset of one row

LeetCode 495 note: each attack contributes min(gap to the next attack, duration), and the last one contributes the full duration