Easy

LeetCode 682 note: keep valid scores on a stack; C removes the last, D adds its double, and + adds the sum of the last two

LeetCode 680 note: at the first mismatch the only options are deleting the left or the right character; check whether either remainder is a palindrome

LeetCode 674 note: one pass that extends the run while strictly increasing and resets to 1 otherwise, updating the maximum after the loop too

LeetCode 671 note: the root is the minimum; the second minimum is the smallest value larger than the root, and only subtrees equal to the root need searching

LeetCode 661 note: each cell becomes the floored average of the existing cells in its 3 × 3 neighborhood, written into a new matrix

LeetCode 657 note: returning to the origin means as many U as D and as many L as R

LeetCode 653 note: traverse while keeping a set of seen values; finding k - val in it means a pair exists

LeetCode 645 note: use sums: missing = sum of 1..n - sum of the set, duplicate = array sum - sum of the set

LeetCode 637 note: level-order traversal, dividing each level’s sum by its count

LeetCode 628 note: after sorting there are only two candidates: the three largest, or the two smallest (negatives) times the largest

LeetCode 605 note: scan left to right and plant in any empty plot whose neighbors are empty; a new flower blocks the next plot, so actually update the array

LeetCode 599 note: map list1’s strings to indices, sum both indices while scanning list2, and keep every string with the minimum sum