Algorithm

To find how much water can be trapped on top of each bar, you need to know the highest bar on the left and the highest bar on the right of that position. The water trapped depends on the smaller of these two heights minus the height of the current bar. If the current bar is taller than both sides, no water can be trapped there. This needs to be done for every bar in the array.

For each bar in the array, find the maximum height bar on its left.
Find the maximum height bar on its right.
Calculate the trapped water on the current bar as min(maxLeft, maxRight) - current height.
Sum all trapped water from each bar to get the total amount of trapped water.