CODE

public class Solution {
  public int Trap(int[] height) {
    if (height.Length == 1) return 0;

    /* break sequence up into 2 types of intervals */
    List<List<int>> fwdIntervals = new List<List<int>>();
    List<List<int>> bkwdIntervals = new List<List<int>>();

    int start = 0, end = height.Length-1;

    /* ignore edge runoff */
    while (height[start] <= height[start+1])
      start++;
    while (height[end] < height[end-1])
      end--;
        
    int i = start;

    while (i < end)
    {
      List<int> interval = new List<int>();
      interval.Add(height[i]);
      int j = i+1;

      if (height[i] == height[j]) {i++; continue;}

      /* forward interval: lg->small->lg */
      else if (height[i] > height[j])
      {
        while (j <= end && height[i] > height[j])
        {
          interval.Add(height[j]);
          j++;
        }
        /* passed the end w/o finding lg - runoff!
            clear the interval and move on */
        if (j > end || (j < end && height[j] == height[j+1])) 
        { 
          i++; 
          interval.Clear(); 
        }
        /* otherwise add the interval to list of forwards */
        else
        { 
            interval.Add(height[j]); 
            fwdIntervals.Add(interval); 
          i = j;
        }
      }
      /* backward interval: sm (after lg) ->lg */
      else
       {
        while (j <= end && height[i] < height[j])
        {
          interval.Add(height[j]);
          j++;
        }
        bkwdIntervals.Add(interval); i = j-1;
      }
    }

    int water = 0;
    /* forward interval water: # of midsection bars times
        left border height minus sum of midsection bars */
    foreach(var f in fwdIntervals)
    {
      int midSum = 0;
      for (int k = 1; k < f.Count-1; k++)
      midSum += f[k];
      int amount = (f.Count-2) * f[0] - midSum;
      water += amount;
    }
    /* backward interval water: sum of right border minus
        current height for all bars up to left border */
    foreach (var b in bkwdIntervals)
    {
      int e = b.Count-1;
      int amount = 0;
      for (int k = 0; k < e; k++)
        amount += (b[e] - b[k]);
      water += amount;
    }
    /* total water: forward + backward water */
    return water;
  }
}