堆棧和隊列的java實現

1.堆棧

堆棧是一種後進先出(LIFO)的數據結構。主要要實現的方法有判斷堆棧是否爲空，輸出堆棧的頂部元素，添加元素，刪除元素，創建一個包含這幾個方法的接口如下:

public interface Stack {

    public boolean isEmpty();             //判斷堆棧是否爲空
    public Object peek();                 //返回堆棧的頂部元素
    public void push(Object theElement);   //從頂部添加元素
    public Object pop();                  //刪除頂部元素並返回被刪除的元素

}

1.數組實現

import java.lang.reflect.Array;
import java.util.EmptyStackException;

public class ArrayStack implements Stack{

    int top;            //current top of stack
    Object element[];    //element array

    /**create a stack with the given initial capacity
      *@throws IlleagalArgumentException when initialCapacity<1*/
    public ArrayStack(int initialCapacity){
        if(initialCapacity<1){
            throw new IllegalArgumentException ("initialCapacity must be >=1");
        }
        element=new Object[initialCapacity];
        top=-1;
    }

    /**create a stack with initial capacity 10*/
    public ArrayStack(){
        this(10);
    }

    /** @return true iff stack is empty*/
    @Override
    public boolean isEmpty() {
        // TODO Auto-generated method stub
        return top==-1;
    }

    /** @return top element of stack
     *  @throw EmptyStackException when the stack is empty*/
    @Override
    public Object peek() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        return element[top];
    }

    /** add the element to the top of stack*/
    @Override
    public void push(Object theElement) {
        // TODO Auto-generated method stub

        //increase array size if necessary
        if(top==element.length-1){
            element=changeArrayLength1D(element,2*element.length);
        }
        //put the element at the top of stack
        element[++top]=theElement;
    }

    private Object[] changeArrayLength1D(Object[] element2, int i) {
        // TODO Auto-generated method stub
        if(element2.length>i){
            throw new IllegalArgumentException("new length too small");
        }
        Object newArray[]=(Object[])Array.newInstance(element2.getClass().getComponentType(), i);
        System.arraycopy(element2, 0, newArray, 0, element2.length);
        return newArray;
    }

    /** @return top element of stack
     *  remove top element of stack
     *  @throw EmptyStackException when the stack is empty*/
    @Override
    public Object pop() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        Object topElement=element[top];
        element[top--]=null;   //enable garbage collection
        return topElement;
    }

    /** @return string converted from stack*/
    public String toString(){
        StringBuilder s=new StringBuilder("[");
        for(int i=top;i>=0;i--){
            s.append(element[i].toString()+",");
        }
        s.append("]");
        return s.toString();
    }

}

構造方法的複雜度是O(n),isEmpty(),peek()，pop方法的複雜度爲O(1),push的複雜度是O(n)(當不需要增加容量時爲O(1)),toString()方法的複雜度爲O(n)。

2.鏈式實現

import java.util.EmptyStackException;

class ChainNode{
    Object element;
    ChainNode next;   

    ChainNode(){}

    ChainNode(Object element){
        this.element=element;
    }

    ChainNode(Object element,ChainNode next){
        this.element=element;
        this.next=next;
    }
}

public class LinkedStack implements Stack{

    protected ChainNode topNode;

    public LinkedStack(int initialCapacity){}

    public LinkedStack(){}


    @Override
    public boolean isEmpty() {
        // TODO Auto-generated method stub
        return topNode==null;
    }

    @Override
    public Object peek() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        return topNode.element;
    }

    @Override
    public void push(Object theElement) {
        // TODO Auto-generated method stub
        topNode=new ChainNode(theElement,topNode);
    }

    @Override
    public Object pop() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        Object topElement=topNode.element;
        topNode=topNode.next;
        return topElement;
    }

    public String toString(){
        StringBuilder s=new StringBuilder("[");
        ChainNode p=topNode;
        while(p!=null){
            s.append(p.element.toString()+",");
            p=p.next;
        }
        s.append("]");
        return s.toString();
    }
}

除了toString()方法複雜度爲O(n)外，其餘方法複雜度都爲O(1)。

2.隊列

隊列是一種先進先出(FIFO)的數據結構。主要要實現的方法和堆棧是差不多的，創建一個隊列接口如下:

public interface Queue {

    public boolean isEmpty();
    public Object getFrontElement();//得到隊列前端元素
    public Object getRearElement(); //得到隊列末端元素
    public void put(Object theElement);
    public Object remove();
}

1.數組實現

import java.lang.reflect.Array;

public class ArrayQueue implements Queue{

    private int front,rear;
    private Object[] element;

    public ArrayQueue(int initialCapacity){
        front=0;
        rear=0;
        element=new Object[initialCapacity];
    }

    public ArrayQueue(){
        this(10);
    }

    @Override
    public boolean isEmpty() {
        // TODO Auto-generated method stub
        return rear==front;
    }

    @Override
    public Object getFrontElement() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        return element[front];
    }

    @Override
    public Object getRearElement() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        int r=0;
        if(rear>0){
            r=rear-1;
        }
        else
            r=element.length-1;
        return element[r];
    }

    @Override
    public void put(Object theElement) {
        // TODO Auto-generated method stub

        if((rear+1)%element.length==front){
            Object newQueue[]=(Object[])Array.newInstance(element.getClass().getComponentType(), 2*element.length);
            if(front==0)
                System.arraycopy(element, front, newQueue, front, element.length-1);
            else{
                System.arraycopy(element, front, newQueue, front, element.length-front);
                System.arraycopy(element, 0, newQueue, element.length, front-1);
            }
            rear=front+element.length;
            element=newQueue;
            element[rear-1]=theElement;
        }
        else{
            element[rear]=theElement;
            rear=(rear+1)%element.length;
        }
    }

    @Override
    public Object remove() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        Object removedElement=element[front];
        front=(front+1)%element.length;     
        return removedElement;
    }

    public String toString(){
        StringBuilder s=new StringBuilder("[");
        if(front<rear){
            for(int i=front;i<rear;i++){
                s.append(element[i].toString()+",");
            }
        }
        else{
            for(int i=front;i<element.length;i++){
                s.append(element[i].toString()+",");
            }
            for(int i=0;i<rear;i++){
                s.append(element[i].toString()+",");
            }
        }
        s.append("]");
        if(!isEmpty()){
            s.delete(s.length()-2,s.length()-1);
        }
        return s.toString();
    }
}

構造方法複雜度爲O(n),put(),toString()方法複雜度爲O(n),其餘方法複雜度爲O(1)。

2.鏈式實現

import java.util.EmptyStackException;

class ChainNode{
    Object element;
    ChainNode next;
    ChainNode(){}
    ChainNode(Object element){
        this.element=element;
    }
    ChainNode(Object element,ChainNode next){
        this.element=element;
        this.next=next;
    }
}

public class LinkedQueue implements Queue{

    ChainNode front,rear;
    public LinkedQueue(int initialCapacity){}
    public LinkedQueue(){}

    @Override
    public boolean isEmpty() {
        // TODO Auto-generated method stub
        return front==null;
    }

    @Override
    public Object getFrontElement() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        return front.element;
    }

    @Override
    public Object getRearElement() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        return rear.element;
    }

    @Override
    public void put(Object theElement) {
        // TODO Auto-generated method stub
        ChainNode p=new ChainNode(theElement,null);
        if(front==null)
            front=p;
        else
            rear.next=p;
        rear=p;
    }

    @Override
    public Object remove() {
        // TODO Auto-generated method stub
        if(isEmpty()){
            throw new EmptyStackException();
        }
        Object removedElement=front.element;
        front=front.next;
        if(isEmpty())
            rear=null;        //enable garbage collection
        return removedElement;
    }

    public String toString(){
        StringBuilder s=new StringBuilder("[");
        ChainNode p=front;
        while(p!=null){
            s.append(p.element.toString()+",");
            p=p.next;
        }
        s.append("]");
        if(!isEmpty()){
            s.delete(s.length()-2,s.length()-1);
        }
        return s.toString();
    }
    public static void main(String args[]){
        LinkedQueue s=new LinkedQueue();
        System.out.println(s.isEmpty());
        for(int i=1;i<8;i++){
            s.put(new Integer(i));
        }
        System.out.println(s.isEmpty());
        System.out.println(s.toString());
        for(int i=1;i<6;i++){
            s.remove();
        }
        System.out.println(s.isEmpty());
        System.out.println(s.toString());
        for(int i=1;i<4;i++){
            s.put(new Integer(i));
        }
        System.out.println(s.toString());
        for(int i=1;i<5;i++){
            s.put(new Integer(i));
        }
        System.out.println(s.toString());
        for(int i=1;i<5;i++){
            s.put(new Integer(i));
        }
        System.out.println(s.toString());
        System.out.println(s.getFrontElement());
        System.out.println(s.getRearElement());
    }

}

toString()方法複雜度爲O(n),其餘方法複雜度爲O(1)。

3.總結

用數組實現比鏈式實現的方法少用內存而且時間更快，但是如果存在多個堆棧或隊列，而它們的長度總和是一定的，但每個數組的範圍都差不多要到從0到它們的長度總和，這個時候使用鏈式實現方法要更好。