Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.
With height = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1], the answer is 6. Water fills the valleys between taller bars. At index 2, the bar height is 0, but water fills up to height 1 (limited by the left maximum).
Think about any single position: how much water can it hold? It depends on the tallest bar to its left and right.
Constraints: , heights from to .