using System;
using System.Collections.Generic;
using System.Linq;
 
public class Test
{
	public static void Main()
	{
		var f = new int[] {2, 0, 2};
		Console.WriteLine("{0}", GetWater(f.ToList()));
	}
 
public static int GetWater(List<int> h)
{
    if (h == null || h.Count < 2) return 0;
    int n = h.Count;
    var left = new int[n];
    var right = new int[n];
 
	int held = 0;
 
	left[0] = h[0];
	for (int i = 1; i < n; i++)
	   left [i] = Math.Max(left[i-1], h[i]);
 
	   right[n-1] = h[n-1];
	    for (int i = n-2; i >= 0; i--)
	       right[i] = Math.Max(right[i+1], h[i]);
 
	for (int i =0; i < n; i++)
	    held += Math.Min(left[i], right[i]) - h[i];
 
	return held;
}
}

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