Java 8 Solution:

public static long largestRectangle(List<Integer> h) {
                //1. Iterate the list of height. Set the max height of the building as first element.
                // then check the height difference to the adjacent building.
                // If the height is same or more, then go to next building.
                //Each time find the max rectangle and update the value.
                long maxRectangle = 0L;

                for(int i = 0; i < h.size(); i++){
                        int maxHt = h.get(i);
                        int maxlen = 1;
                        for(int j = i+1; j < h.size(); j++){
                                if(h.get(j) - h.get(j-1) < 0){
                                        if(maxHt > h.get(j))maxHt = h.get(j);
                                }
                                maxlen+= 1;
                                if(maxRectangle < maxHt*maxlen) maxRectangle = maxHt*maxlen;
                        }
                }
                return maxRectangle;
        }