A data structure is a way to store and organize data in order to facilitate access and modifications.
We use loop invariants to help us understand why an algorithm is correct. We must show three things about a loop invariant:
Initialization: It is true prior to the first iteration of the loop.
Maintenance: If it is true before an iteration of the loop, it remains true before the next iteration.
Termination: When the loop terminates, the invariant gives us a useful property that helps show that the algorithm is correct.
原文：http://www.cnblogs.com/Patt/p/5814221.html