My naive solution to a programmer puzzle I saw on hackernews. https://news.ycombinator.com/item?id=6639839 edit: Added the smart way of doing it too

Puddles.java

public class Puddles
{		
	public static void main(String[] args) throws Exception
	{
		//set heights, need 2 or more
		int[] groundheights = {2, 5, 1, 2, 3, 4, 7, 7, 6};
		String gh = "Ground heights: ";
		for (int i = 0; i<groundheights.length; i++)
		{
			if (i == 0) {
				gh += groundheights[i];
			} else {
				gh += ", " + groundheights[i];
			}
		}
		System.out.println(gh);
		
		System.out.println("water: " + doDumb(groundheights));
		System.out.println("water: " + doSmart(groundheights));
	}
	
	public static int doDumb(int[] groundheights)
	{
		//calculate water sheding to the left
		int[] watershedleftheights = new int[groundheights.length];
		watershedleftheights[0] = groundheights[0];
		for (int i = 1; i<groundheights.length; i++)
		{
			if (groundheights[i] > watershedleftheights[i-1])
			{
				watershedleftheights[i] = groundheights[i];
			} else {
				watershedleftheights[i] = watershedleftheights[i-1];
			}
		}
		
		//calculate water shedding to the right
		int[] watershedrightheights = new int[groundheights.length];
		watershedrightheights[groundheights.length-1] = groundheights[groundheights.length-1];
		for (int i = groundheights.length-2; i>-1; i--)
		{
			if (groundheights[i] > watershedrightheights[i+1])
			{
				watershedrightheights[i] = groundheights[i];
			} else {
				watershedrightheights[i] = watershedrightheights[i+1];
			}
		}
		
		//calc the volumes
		int watervolume = 0;
		for (int i = 0; i<groundheights.length;i++)
		{
			int lowestwatershed;
			if (watershedleftheights[i] < watershedrightheights[i])
			{
				lowestwatershed = watershedleftheights[i];
			} else {
				lowestwatershed = watershedrightheights[i];

			}
			
			if (lowestwatershed > groundheights[i])
			{
				watervolume += lowestwatershed - groundheights[i];
			}
		}
		return watervolume;
	}
	
	public static int doSmart(int[] groundheights)
	{
		//set the trace from left
		int shedleftpointer = 0;
		int shedlefttraceheight = 0;
		
		//set the trace from right
		int shedrightpointer = groundheights.length-1;
		int shedrighttraceheight = 0;
		
		int watervolume = 0;
		//until the tracers overlap
		while (shedleftpointer <= shedrightpointer)
		{
			//update the lower tracer, to prevent adding too much to watervolume
			if (shedlefttraceheight < shedrighttraceheight)
			{
				//do left
				if (shedlefttraceheight > groundheights[shedleftpointer])
				{
					//if watershed tracer larger than the current groundheight
					//then water will pool in the cell, so add the volume 
					watervolume += shedlefttraceheight - groundheights[shedleftpointer];
				} else {
					//otherwise bump up the trace height or set it the same
					shedlefttraceheight = groundheights[shedleftpointer];
				}
				shedleftpointer++;
			} else {
				//do right
				if (shedrighttraceheight > groundheights[shedrightpointer])
				{
					watervolume += shedrighttraceheight - groundheights[shedrightpointer];
				} else {
					shedrighttraceheight = groundheights[shedrightpointer];
				}
				shedrightpointer--;
			}
		}
		
		return watervolume;
	}
}