Two Pointers

Start pointers at both ends, area is shorter height times width, and always move the shorter side inward.

Water is limited by the shorter wall, so moving the shorter pointer inward is the only move that can possibly increase the area.

Two pointers turns an O(n squared) nested-loop scan into a single O(n) sweep using two moving indexes instead of one.

Reverse the flow: BFS/DFS inward from each ocean's border going uphill, mark each ocean's reachable set, and return the cells in both sets.

Keep a boundary for the next 0, sweep once, and swap every 0 you find up to that boundary.

Sort, fix one, then two-pointer the rest; skip duplicates at every level.

Same two-pointer sweep as 3Sum, but instead of requiring an exact match, remember the closest sum and chase the target.

When left plus right is under target, every pair between them also qualifies, so add the whole span, not just one.

Water over a bar is min(leftMax, rightMax) minus the bar height, and two pointers add it from the shorter side.

Process the side with the smaller boundary wall; the water above a bar equals that side's running max minus the bar's own height.