Medium

LeetCode 240 note: start at the top-right corner, moving left when larger and down when smaller, eliminating a row or column each step

LeetCode 229 note: at most two values can exceed n / 3; Boyer-Moore keeps two candidates and a second pass verifies their counts

LeetCode 216 note: use only 1 to 9, each at most once, choosing exactly k numbers that sum to n

LeetCode 151 note: split() with no argument treats runs of spaces as one separator and drops leading and trailing spaces; reverse and join with single spaces

LeetCode 204 note: the sieve of Eratosthenes, crossing out multiples of each prime p starting at p², with the outer loop only up to √n

LeetCode 189 note: take k %= n, then reverse three times: the whole array, the first k, and the last n - k, with O(1) extra space

LeetCode 162 note: nums[mid] < nums[mid + 1] guarantees a peak to the right (rising until the edge at worst), otherwise one at or left of mid

LeetCode 134 note: if total gas is short there is no answer; otherwise scan, and whenever the tank goes negative move the start to the next station

LeetCode 137 note: with everything else appearing three times, count the 1s at each bit modulo 3; the leftover bits form the answer, minding negatives

LeetCode 106 note: the last post-order value is the root; taking roots from the end of the post-order, build the right subtree before the left

LeetCode 93 note: cut into exactly 4 pieces, each 1 to 3 digits long, valued 0 to 255, with no leading zero except 0 itself

LeetCode 647 note: expand around centers, counting one palindromic substring for each successful expansion