A stack is a data structure that is often likened to a stack of plates. If you have just washed a plate, you put it on top of the stack. When you need a plate, you take it off the top of the stack. So the last plate to be added to the stack will be the first to be removed from the stack. Thus, a stack is a last in, first out (LIFO) structure:
The preceding figure depicts a stack of plates. Adding a plate to the pile is only possible by leaving that plate on top of the pile. To remove a plate from the pile of plates means to remove the plate that is on top of the pile.
There are two primary operations that are done on stacks: push and pop. When an element is added to the top of the stack, it is pushed onto the stack. When an element is taken off the top of the stack, it is popped off the stack. Another operation which is used sometimes is peek, which makes it possible to see the element on the stack without popping it off.
Stacks are used for a number of things. One very common usage...