# 实现双向链表

``````package com.dfsn.cloud.eureka;

// 头节点
private Node first;

// 尾节点
private Node last;

// 元素个数
private int size = 0;

// 增加元素
if (first == null) {
Node newNode = new Node(t, null, null);
first = newNode;
last = newNode;
} else {
Node newLast = new Node(t, null, last);
last.next = newLast;
last = newLast;
}
size++;
}

// 返回元素个数
public int size() {
return size;
}

// 遍历链表
public void show() {
if (first == null) {
throw new RuntimeException("链表为空");
} else {
Node temp = first;
while (true) {
System.out.println(temp);
if (temp.next == null) {
break;
}
temp = temp.next;
}

}
}

// 根据下标获取节点
public T get(int index) {
return get(index, "").item;
}

private Node get(int index, String s) {
if (first == null) {
throw new RuntimeException("链表为空，没有对应节点");
} else if (index >= size) {
throw new RuntimeException("链表长度为：" + size + ",index为" + (size - 1));
} else if (index < 0) {
throw new RuntimeException("下标不能<0");
} else {
int avg = size / 2;
if (index < avg) {
Node temp = first;
for (int i = 0; i < index; i++) {
temp = temp.next;
}
return temp;
} else {
Node temp = last;
for (int i = size - 1; i > index; i--) {
temp = temp.prev;
}
return temp;
}
}
}

// 在指定位置插入
public void add(int index, T t) {
if (index > size) {
throw new RuntimeException("链表长度为：" + size + ",index为" + size);
} else if (index < 0) {
throw new RuntimeException("下标不能<0");
} else if (index == 0) {
Node newNode = new Node(t, first, null);
newNode.next = first;
first = newNode;
} else if (index == size) {
Node newNode = new Node(t, null, last);
last.next = newNode;
last = newNode;
} else {
// 原来位置的node
Node node = get(index, "");
Node newNode = new Node(t, node, node.prev);
node.prev.next = newNode;
node.prev = newNode;
}
size++;
}

// 更改指定位置的元素
public void set(int index, T t) {
if (index >= size) {
throw new RuntimeException("链表长度为：" + size + ",index为" + (size - 1));
} else if (index < 0) {
throw new RuntimeException("下标不能<0");
} else {
Node node = get(index, "");
node.item = t;
}
}

// 删除指定位置的元素
public void remove(int index) {
if (index >= size) {
throw new RuntimeException("链表长度为：" + size + ",index为" + (size - 1));
} else if (index < 0) {
throw new RuntimeException("下标不能<0");
} else if (index == 0) {
first = first.next;
first.prev = null;
} else if (index == size - 1) {
last = last.prev;
last.next = null;
} else {
Node node = get(index, "");
Node next = node.next;
Node prev = node.prev;
prev.next = next;
next.prev = prev;
}
size--;
}

// 返回头节点
public T getFirst() {
return first.item;
}

// 返回尾节点
public T getLasts() {
return last.item;
}

// 节点对象
class Node {
private T item;
private Node next;
private Node prev;

public Node(T item, Node next, Node prev) {
this.item = item;
this.next = next;
this.prev = prev;
}

@Override
public String toString() {
return "Node [item=" + item + "]";
}

}
}

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