之前的數據結構都是在動態數組基礎上實現的。而另外一種作爲實現ADT(抽象數據類型)的基礎,鏈表也是需要掌握的,具體實現爲
public class LinkedList<E> {
private class Node{
public E e;
public Node next;
public Node(E e, Node next){
this.e = e;
this.next = next;
}
public Node(E e){
this(e, null);
}
public Node(){
this(null, null);
}
@Override
public String toString(){
return e.toString();
}
}
private Node dummyHead;
private int size;
public LinkedList(){
dummyHead = new Node();
size = 0;
}
// 獲取鏈表中的元素個數
public int getSize(){
return size;
}
// 返回鏈表是否爲空
public boolean isEmpty(){
return size == 0;
}
// 在鏈表的index(0-based)位置添加新的元素e
// 在鏈表中不是一個常用的操作,練習用:)
public void add(int index, E e){
if(index < 0 || index > size)
throw new IllegalArgumentException("Add failed. Illegal index.");
Node prev = dummyHead;
for(int i = 0 ; i < index ; i ++)
prev = prev.next;
prev.next = new Node(e, prev.next);
size ++;
}
// 在鏈表頭添加新的元素e
public void addFirst(E e){
add(0, e);
}
// 在鏈表末尾添加新的元素e
public void addLast(E e){
add(size, e);
}
// 獲得鏈表的第index(0-based)個位置的元素
// 在鏈表中不是一個常用的操作,練習用:)
public E get(int index){
if(index < 0 || index >= size)
throw new IllegalArgumentException("Get failed. Illegal index.");
Node cur = dummyHead.next;
for(int i = 0 ; i < index ; i ++)
cur = cur.next;
return cur.e;
}
// 獲得鏈表的第一個元素
public E getFirst(){
return get(0);
}
// 獲得鏈表的最後一個元素
public E getLast(){
return get(size - 1);
}
// 修改鏈表的第index(0-based)個位置的元素爲e
// 在鏈表中不是一個常用的操作,練習用:)
public void set(int index, E e){
if(index < 0 || index >= size)
throw new IllegalArgumentException("Set failed. Illegal index.");
Node cur = dummyHead.next;
for(int i = 0 ; i < index ; i ++)
cur = cur.next;
cur.e = e;
}
// 查找鏈表中是否有元素e
public boolean contains(E e){
Node cur = dummyHead.next;
while(cur != null){
if(cur.e.equals(e))
return true;
cur = cur.next;
}
return false;
}
// 從鏈表中刪除index(0-based)位置的元素, 返回刪除的元素
// 在鏈表中不是一個常用的操作,練習用:)
public E remove(int index){
if(index < 0 || index >= size)
throw new IllegalArgumentException("Remove failed. Index is illegal.");
Node prev = dummyHead;
for(int i = 0 ; i < index ; i ++)
prev = prev.next;
Node retNode = prev.next;
prev.next = retNode.next;
retNode.next = null;
size --;
return retNode.e;
}
// 從鏈表中刪除第一個元素, 返回刪除的元素
public E removeFirst(){
return remove(0);
}
// 從鏈表中刪除最後一個元素, 返回刪除的元素
public E removeLast(){
return remove(size - 1);
}
// 從鏈表中刪除元素e
public void removeElement(E e){
Node prev = dummyHead;
while(prev.next != null){
if(prev.next.e.equals(e))
break;
prev = prev.next;
}
if(prev.next != null){
Node delNode = prev.next;
prev.next = delNode.next;
delNode.next = null;
size --;
}
}
@Override
public String toString(){
StringBuilder res = new StringBuilder();
Node cur = dummyHead.next;
while(cur != null){
res.append(cur + "->");
cur = cur.next;
}
res.append("NULL");
return res.toString();
}
}
這裏的鏈表使用了虛擬頭結點便於進行刪除等操作,在鏈表的基礎上,我們接下來可以繼續實現棧和隊列等結構。
首先,一個棧應實現的操作爲(接口)
public interface Stack<E> {
int getSize();
boolean isEmpty();
void push(E e);
E pop();
E peek();
}
在有了鏈表後,我們可以方便的實現這些方法
public class LinkedListStack<E> implements Stack<E> {
private LinkedList<E> list;
public LinkedListStack(){
list = new LinkedList<>();
}
@Override
public int getSize(){
return list.getSize();
}
@Override
public boolean isEmpty(){
return list.isEmpty();
}
@Override
public void push(E e){
list.addFirst(e);
}
@Override
public E pop(){
return list.removeFirst();
}
@Override
public E peek(){
return list.getFirst();
}
@Override
public String toString(){
StringBuilder res = new StringBuilder();
res.append("Stack: top ");
res.append(list);
return res.toString();
}
public static void main(String[] args) {
LinkedListStack<Integer> stack = new LinkedListStack<>();
for(int i = 0 ; i < 5 ; i ++){
stack.push(i);
System.out.println(stack);
}
stack.pop();
System.out.println(stack);
}
}
而要實現隊列的話,則需要定義隊列的以下接口
public interface Queue<E> {
int getSize();
boolean isEmpty();
void enqueue(E e);
E dequeue();
E getFront();
}
具體實現爲
public class LinkedListQueue<E> implements Queue<E> {
private class Node{
public E e;
public Node next;
public Node(E e, Node next){
this.e = e;
this.next = next;
}
public Node(E e){
this(e, null);
}
public Node(){
this(null, null);
}
@Override
public String toString(){
return e.toString();
}
}
private Node head, tail;
private int size;
public LinkedListQueue(){
head = null;
tail = null;
size = 0;
}
@Override
public int getSize(){
return size;
}
@Override
public boolean isEmpty(){
return size == 0;
}
@Override
public void enqueue(E e){
if(tail == null){
tail = new Node(e);
head = tail;
}
else{
tail.next = new Node(e);
tail = tail.next;
}
size ++;
}
@Override
public E dequeue(){
if(isEmpty())
throw new IllegalArgumentException("Cannot dequeue from an empty queue.");
Node retNode = head;
head = head.next;
retNode.next = null;
if(head == null)
tail = null;
size --;
return retNode.e;
}
@Override
public E getFront(){
if(isEmpty())
throw new IllegalArgumentException("Queue is empty.");
return head.e;
}
@Override
public String toString(){
StringBuilder res = new StringBuilder();
res.append("Queue: front ");
Node cur = head;
while(cur != null) {
res.append(cur + "->");
cur = cur.next;
}
res.append("NULL tail");
return res.toString();
}
}
這裏使用了head和tail兩個節點來維護隊列,但這樣會有浪費空間的問題,在之前基於動態數組時,我們使用了循環隊列,現在我們用基於鏈表的方法實現
public class LoopQueue<E> implements Queue<E> {
private E[] data;
private int front, tail;
private int size; // 有興趣的同學,在完成這一章後,可以思考一下:
// LoopQueue中不聲明size,如何完成所有的邏輯?
// 這個問題可能會比大家想象的要難一點點:)
public LoopQueue(int capacity){
data = (E[])new Object[capacity + 1];
front = 0;
tail = 0;
size = 0;
}
public LoopQueue(){
this(10);
}
public int getCapacity(){
return data.length - 1;
}
@Override
public boolean isEmpty(){
return front == tail;
}
@Override
public int getSize(){
return size;
}
@Override
public void enqueue(E e){
if((tail + 1) % data.length == front)
resize(getCapacity() * 2);
data[tail] = e;
tail = (tail + 1) % data.length;
size ++;
}
@Override
public E dequeue(){
if(isEmpty())
throw new IllegalArgumentException("Cannot dequeue from an empty queue.");
E ret = data[front];
data[front] = null;
front = (front + 1) % data.length;
size --;
if(size == getCapacity() / 4 && getCapacity() / 2 != 0)
resize(getCapacity() / 2);
return ret;
}
@Override
public E getFront(){
if(isEmpty())
throw new IllegalArgumentException("Queue is empty.");
return data[front];
}
private void resize(int newCapacity){
E[] newData = (E[])new Object[newCapacity + 1];
for(int i = 0 ; i < size ; i ++)
newData[i] = data[(i + front) % data.length];
data = newData;
front = 0;
tail = size;
}
@Override
public String toString(){
StringBuilder res = new StringBuilder();
res.append(String.format("Queue: size = %d , capacity = %d\n", size, getCapacity()));
res.append("front [");
for(int i = front ; i != tail ; i = (i + 1) % data.length){
res.append(data[i]);
if((i + 1) % data.length != tail)
res.append(", ");
}
res.append("] tail");
return res.toString();
}
}