Java--practice---0

#Sentinel node

Https://en.wikipedia.org/wiki/Sentinel_node#First_version_using_NULL_as_an_end-of-list_indicator

# Reference Code, note import two libraries

Reference: http://blog.csdn.net/u010005281/article/details/50812328

/****************************************************************************** * Compilation:javac   Linkedstack.java * Execution:java Linkedstack < Input.txt * Dependencies:StdIn.java Stdout.java * Data files: Https://algs4.cs.princeton.edu/13stacks/tobe.txt* * A generic stack, implemented using A linked list. Each of the stack * element is of type Item. * *% more tobe.txt * to being or not to-be--that--is * * *% java linkedstack < Tobe.txt * Or is (2 left on stack) * ******************************************************************************/ImportEdu.princeton.cs.algs4.StdOut;Importedu.princeton.cs.algs4.StdIn;ImportJava.util.Iterator;Importjava.util.NoSuchElementException;/*** The {@codeLinkedstack} class represents a Last-in-first-out (LIFO) stack of * generic items. * It supports the usual <em>push</em> <em>pop</em> operations, along with methods * for Peek ing at the top item, testing if the stack was empty, and iterating through * the items in LIFO order. * <p> * This implementation uses a singly linked list with a non-static nested class for * linked-list nodes. See {@linkStack} for a version that uses a static nested class. * the <em>push</em>, <em>pop</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em> * Operations all take constant time in the worst case. * <p> * For additional documentation, * see <a href= "Https://algs4.cs.princeton.edu/13stacks">section 1.3</a> of * <i>algorithms, 4th edition</i> by Robert Sedgewick and Kevin Wayne. * *  @authorRobert Sedgewick *@authorKevin Wayne*/ Public classLinkedstack<item>ImplementsIterable<item> {    Private intN//size of the stack    PrivateNode first;//Top of Stack//Helper Linked List class    Private classNode {Privateitem Item; PrivateNode Next; }    /*** Initializes an empty stack. */     PublicLinkedstack () { First=NULL; N= 0; assertcheck (); }    /*** Is this stack empty? * @returntrue if this stack is empty; false otherwise*/     Public BooleanIsEmpty () {returnFirst = =NULL; }    /*** Returns the number of items in the stack. * @returnThe number of items in the stack*/     Public intsize () {returnN; }    /*** Adds The item to this stack. * @paramitem the item to add*/     Public voidpush (item item) {Node Oldfirst=First ; First=NewNode (); First.item=item; First.next=Oldfirst; N++; assertcheck (); }    /*** Removes and returns the item most recently added to this stack. * @returnThe item most recently added *@throwsjava.util.NoSuchElementException If this stack is empty*/     PublicItem Pop () {if(IsEmpty ())Throw NewNosuchelementexception ("Stack underflow"); Item Item= First.item;//Save item to returnfirst = First.next;//Delete First noden--; assertcheck (); returnItem//return the saved item    }    /*** Returns (but does not remove) the item is the recently added to this stack. * @returnThe item most recently added to this stack *@throwsjava.util.NoSuchElementException If this stack is empty*/     PublicItem Peek () {if(IsEmpty ())Throw NewNosuchelementexception ("Stack underflow"); returnFirst.item; }    /*** Returns A string representation of this stack. * @returnthe sequence of items in the stack in LIFO order, separated by spaces*/     PublicString toString () {StringBuilder s=NewStringBuilder ();  for(Item Item: This) S.append (item+ " "); returns.tostring (); }           /*** Returns An iterator to this stack, that iterates through, the items in LIFO order. * @returnAn iterator to this stack, the iterates through the items in LIFO order. */     PublicIterator<item>iterator () {return NewListiterator (); }    //An iterator, doesn ' t implement remove () since it ' s optional    Private classListiteratorImplementsIterator<item> {        PrivateNode current =First ;  Public BooleanHasnext () {returnCurrent! =NULL; }         Public voidRemove () {Throw Newunsupportedoperationexception (); }         PublicItem Next () {if(!hasnext ())Throw Newnosuchelementexception (); Item Item=Current.item; Current=Current.next; returnitem; }    }    //Check internal invariants    Private BooleanCheck () {//check a few properties of instance variable ' first '        if(N < 0) {            return false; }        if(n = = 0) {            if(First! =NULL)return false; }        Else if(n = = 1) {            if(First = =NULL)return false; if(First.next! =NULL)return false; }        Else {            if(First = =NULL)return false; if(First.next = =NULL)return false; }        //Check internal consistency of instance variable n        intNumberofnodes = 0;  for(Node x = first; X! =NULL&& numberofnodes <= N; x =x.next) {numberofnodes++; }        if(Numberofnodes! = N)return false; return true; }    /*** Unit tests the {@codelinkedstack} data type. *     * @paramargs the command-line arguments*/     Public Static voidMain (string[] args) {Linkedstack<String> stack =NewLinkedstack<string>();  while(!Stdin.isempty ()) {String Item=stdin.readstring (); if(!item.equals ("-") ) Stack.push (item); Else if(!Stack.isempty ()) Stdout.print (Stack.pop ()+ " "); } stdout.println ("(" + stack.size () + "left on stack)"); }}

Java--practice---0