Data Structures and Algorithm Analysis - Computer Science at ...

120 Chap. 4 Lists, Stacks, and Queues/** Linked stack implementation */class LStack implements Stack {private Link top;// Pointer to first elementprivate int size;// Number of elements/** Constructors */public LStack() { top = null; size = 0; }public LStack(int size) { top = null; size = 0; }/** Reinitialize stack */public void clear() { top = null; size = 0; }/** Put "it" on stack */public void push(E it) {top = new Link(it, top);size++;}/** Remove "it" from stack */public E pop() {assert top != null : "Stack is empty";E it = top.element();top = top.next();size--;return it;}/** @return Top value */public E topValue() {assert top != null : "Stack is empty";return top.element();}/** @return Stack length */public int length() { return size; }4.2.2 Linked StacksFigure 4.20 Linked stack class implementation.The linked stack implementation is quite simple. The freelist of Section 4.1.2 isan example of a linked stack. Elements are inserted and removed only from thehead of the list. A header node is not used because no special-case code is requiredfor lists of zero or one elements. Figure 4.20 shows the complete linked stackimplementation. The only data member is top, a pointer to the first (top) link nodeof the stack. Method push first modifies the next field of the newly created linknode to point to the top of the stack and then sets top to point to the new linknode. Method pop is also quite simple. Variable temp stores the top nodes’ value,while ltemp links to the top node as it is removed from the stack. The stack isupdated by setting top to point to the next link in the stack. The old top node isthen returned to free store (or the freelist), and the element value is returned.