Thanks!

Let's take the first 2 examples in description:

[5,2,10] should return 3

So if it rains, there will be 3 units of water between 5 and 10 (it will become [5, 5, 10] (any more would just fall to left side, or right side of 10, which we don't care about)

[1,0,5,2,6,3,10] should return 7

So there is space for 1 unit between 1 and 5, 3 units between 5 and 6, and 3 more units between 6 and 10, ending in state [1, 1, 5, 5, 6, 6, 10] (1 + 3 + 3 = 7)

Does that make any sense?