see http://programmingpraxis.com/2013/11/15/twitter-puddle/

puddle.java

package org.b7j0c.puzzles;

// http://programmingpraxis.com/2013/11/15/twitter-puddle/

public final class Puddles {
    public static void main(String[] args) {
        int[] c = {2,5,1,2,3,4,7,7,6,1,1,9,8,8,9,1,1};
        int cl = c.length;
        int w = -1;
        int sub = 0;
        for (int i=1; i < cl; i++) {
            if (c[i] > c[i-1]) { // i is a right wall
                if (w != -1) {
                    sub += c[i-1];
                    if (c[i] >= c[w]) {
                        // vol is height of left wall w * dist to i (right wall) 
                        // minus accumulated columns heights in valley
                        int vol = (c[w] * (i-w-1)) - sub;
                        System.out.printf("from %d to %d, vol %d\n",w,i,vol);
                        sub = 0;
                        w = -1;
                    }
                }
            } else if (c[i] < c[i-1]) { // i-1 is a left wall
                w = i-1;
            } else { // if there is a left wall, add this to valley column heights
                if (w >= 0) sub += c[i-1];
            }
        }
    }
}