Given an array height where height[i] is the height of a vertical line standing at index i, pick two lines that, together with the x-axis, form a container. Return the maximum amount of water that container can hold — min(height[i], height[j]) × (j - i) for the pair you pick. Checking every pair works but wastes time re-deriving what a single pass can rule out: since water level is always capped by the shorter of the two chosen lines, a two-pointer sweep that always advances the shorter side finds the answer in O(n).