What is the trapping rain water problem and how does it use two pointers?

The trapping rain water problem asks how much water can be held between bars of varying heights. The key insight is that water above each bar equals the minimum of the maximum height to its left and maximum height to its right, minus the bar's own height. The two-pointer approach uses left and right pointers with running maximums for each side. Process the side with the smaller maximum because you know the water level there is determined by that side. If the current bar is shorter than its side maximum, the difference is trapped water. ---