Constructive Algorithm are more like a adhoc solution, they are sometimes called as adhoc problem as far as i know. This type of problem doesn't fall on a specific or well known algorithm technique. so the solution are problem dependent. you have to invent your own idea. Simple, constructive algorithm are those which can't be solved by any well known or specific algorihm. And Second Implemention. Well,the name says all. You have to implement your idea. Most of the time you just have to simulate the event which was told in the problems. For better understanding , try to solve more problems of this kind. then you will realize the difference automatically.