Java: Trapping rain water

Difficulty Level: HARD

Problem Statement:

Given an array of N non-negative integers arr[] representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after rain.

Input:

arr[]   = {3, 0, 2, 0, 4}

Output:

7 
We can trap “3 units” of water between 3 and 2, “1 unit” on top of bar 2 and “3 units” between 2 and 4.