Solution for http://habrahabr.ru/post/200190/

gistfile1.cs

using System;
using System.Collections.Generic;

namespace TwitterTest
{

    public class Solver
    {
        private readonly int[] _walls;

        public Solver(int[] walls)
        {
            _walls = walls;
        }

        public int Solve()
        {
            var result = 0;
            var stack = new Stack<Wall>();
            for (var i = 0; i < _walls.Length; i++)
            {
                var currentWaterHeight = 0;
                if (stack.Count > 0 && _walls[i] > stack.Peek().Height)
                {
                    while (stack.Count > 0)
                    {
                        var wall = stack.Peek();

                        result += (i - wall.Index - 1) * Math.Abs(Math.Min(wall.Height, _walls[i]) - currentWaterHeight);
                        
                        if (wall.Height <= _walls[i])
                        {
                            stack.Pop();
                            currentWaterHeight = wall.Height;
                        }
                        else
                        {
                            break;
                        }
                    }
                }
                stack.Push(new Wall(i, _walls[i]));
            }
            return result;
        }

        private class Wall
        {
            public Wall(int index, int height)
            {
                Index = index;
                Height = height;
            }

            public int Index { get; private set; }
            public int Height { get; private set; }
        }
    }
}

gistfile2.cs

using NUnit.Framework;

namespace TwitterTest
{
    [TestFixture]
    public class SolverTests
    {
        [Test]
        public void Test1()
        {
            var array = new[] {2, 5, 1, 2, 3, 4, 7, 7, 6};
            var solver = new Solver(array);

            var result = solver.Solve();

            Assert.That(result, Is.EqualTo(10));
        }

        [Test]
        public void Test2()
        {
            var array = new[] { 2, 5, 1, 3, 1, 2, 1, 7, 7, 6 };
            var solver = new Solver(array);

            var result = solver.Solve();

            Assert.That(result, Is.EqualTo(17));
        }

        [Test]
        public void Test3()
        {
            var array = new[] {1, 2, 3, 4, 5};
            var solver = new Solver(array);

            var result = solver.Solve();

            Assert.That(result, Is.EqualTo(0));
        }

        [Test]
        public void Test4()
        {
            var array = new[] { 1, 0, 0, 0, 1, 2 };
            var solver = new Solver(array);

            var result = solver.Solve();

            Assert.That(result, Is.EqualTo(3));
        }

        [Test]
        public void Test5()
        {
            var array = new[] { 1, 0, 0, 5, 3, 5, 0, 1, 2 };
            var solver = new Solver(array);

            var result = solver.Solve();

            Assert.That(result, Is.EqualTo(7));
        }
    }
}