Task: There is a landscape with hills and pits which have similar square shape. Each position in this landscape has a specified height. Max number of positions is 32000.

Height is between 0 and 32000.

For example: the first position has height 5, the second position has height 2. The landscape can be presented as a collection of heights. {5,2,3,4,5,4,0,3,1}

When rain happens, landscape is filled with water. Water is collected inside pits only between hills. For example: collected 9 squares of water.

You need to:

• implement an application which calculates amount of collected water for any landscapes. Water calculator should implement method long calculateWaterAmount(int[] landscape)
• validate correctness of landscape
• cover this application with tests (TDD)
• use maven to build the application

Solution has O(n) complexity and O(n) memory complexity.

Possible optimizations:

• Validate landscape height inside main loop
• Use one array for leftMaxLevels and rightMaxLevels. Height couldn't be more than 32000 (fit in 2^15) and int in java is 2^32. So it's possible to store to numbers in one int with certain manipulations.
• Or even don't use additional arrays at all. Use int[] landscape to keep landscape height and leftMaxLevels in first loop. And calculate rightMaxLevels and result in second loop.