Stacks and queues are dynamic sets in which the element removed from the set by the DELETE operation is prespecified. In a stack, the element deleted from the set is the one most recently inserted: the stack implements a last-in, first-out, or LIFO, policy. Similarly, in a queue, the element deleted is always the one that has been in the set for the longest time: the queue implements a first-in, first out, or FIFO, policy. There are several efficient ways to implement stacks and queues on a computer. In this section we show how to use a simple array to implement each.