在 Go 语言中，container/list 包提供了一个双向链表的实现。链表是一种常见的数据结构，适用于频繁插入和删除操作的场景。container/list 包中的链表是双向的，意味着每个元素都包含指向前一个和后一个元素的指针。

力扣：146. LRU 缓存

力扣算法链接：https://leetcode.cn/problems/lru-cache/?envType=study-plan-v2&envId=top-100-liked

请你设计并实现一个满足 LRU (最近最少使用) 缓存 约束的数据结构。
实现 LRUCache 类：
LRUCache(int capacity) 以 正整数 作为容量 capacity 初始化 LRU 缓存。
int get(int key) 如果关键字 key 存在于缓存中，则返回关键字的值，否则返回 -1 。
void put(int key, int value) 如果关键字 key 已经存在，则变更其数据值 value ；如果不存在，则向缓存中插入该组 key-value 。如果插入操作导致关键字数量超过 capacity ，则应该 逐出 最久未使用的关键字。

函数 get 和 put 必须以 O(1) 的平均时间复杂度运行。

输入
[“LRUCache”, “put”, “put”, “get”, “put”, “get”, “put”, “get”, “get”, “get”]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]

输出
[null, null, null, 1, null, -1, null, -1, 3, 4]

解释
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // 缓存是 {1=1}
lRUCache.put(2, 2); // 缓存是 {1=1, 2=2}
lRUCache.get(1); // 返回 1
lRUCache.put(3, 3); // 该操作会使得关键字 2 作废，缓存是 {1=1, 3=3}
lRUCache.get(2); // 返回 -1 (未找到)
lRUCache.put(4, 4); // 该操作会使得关键字 1 作废，缓存是 {4=4, 3=3}
lRUCache.get(1); // 返回 -1 (未找到)
lRUCache.get(3); // 返回 3
lRUCache.get(4); // 返回 4

代码案例：

type Node struct {key   intvalue int
}type LRUCache struct {capacity intlist     *list.Listmp       map[int]*list.Element
}func Constructor(capacity int) LRUCache {return LRUCache{capacity: capacity,list:     list.New(),mp:       make(map[int]*list.Element),}
}func (this *LRUCache) Get(key int) int {if v, ok := this.mp[key]; ok {this.list.MoveToFront(v)return v.Value.(*Node).value}return -1
}func (this *LRUCache) Put(key int, value int) {if v, ok := this.mp[key]; ok {v.Value.(*Node).value = valuethis.list.MoveToFront(v)return}node := &Node{key, value}a := this.list.PushFront(node)this.mp[key] = aif this.list.Len() > this.capacity {tmp := this.list.Back()delete(this.mp, tmp.Value.(*Node).key)this.list.Remove(tmp)}
}

主要结构 List 和 Element

• List: 表示一个双向链表。

type List struct {root Element // sentinel list element, only &root, root.prev, and root.next are usedlen  int     // current list length excluding (this) sentinel element
}

• Element: 表示链表中的一个元素。

type Element struct {next, prev *Elementlist *ListValue any
}

常用方法

1. 初始化链表

使用 list.New() 创建一个新的链表。

func main() {l := list.New()fmt.Printf("%+v\n",l)
}

2. 插入元素

• PushBack(value interface{}) *Element: 在链表尾部插入一个元素。
• PushFront(value interface{}) *Element: 在链表头部插入一个元素。
• InsertBefore(value interface{}, mark *Element) *Element: 在指定元素前插入一个元素。
• InsertAfter(value interface{}, mark *Element) *Element: 在指定元素后插入一个元素。

func main() {l := list.New()l.PushBack(123)l.PushBack("nihao")l.PushFront("你好")l.PushFront(3.1415926)// 遍历for e := l.Front(); e != nil; e = e.Next() {fmt.Printf("%+v\n", e)}
}

通过运行结果可以发现，list其实就是一个环形的双向链表。

3. 删除元素

• Remove(e *Element) interface{}: 删除链表中的指定元素。

func main() {l := list.New()l.PushBack("nihao")a:=l.Remove(l.Back())fmt.Println(a)
}

4. 遍历链表

• Front() *Element: 返回链表的第一个元素。
• Back() *Element: 返回链表的最后一个元素。
• Next() *Element: 返回当前元素的下一个元素。
• Prev() *Element: 返回当前元素的前一个元素。

func main() {l := list.New()l.PushBack(1)l.PushBack(2)l.PushBack(3)// 从前往后遍历for e := l.Front(); e != nil; e = e.Next() {fmt.Println(e.Value)}// 从后往前遍历for e := l.Back(); e != nil; e = e.Prev() {fmt.Println(e.Value)}
}

5. 获取链表长度

• Len() int: 返回链表中元素的个数。

func main() {l := list.New()l.PushBack(1)l.PushBack(2)l.PushBack(3)fmt.Println(l.Len()) // 输出: 3
}

使用场景

• 频繁插入和删除: 链表在插入和删除操作上比数组更高效，尤其是在中间位置。
• 实现队列和栈: 链表可以用来实现队列（FIFO）和栈（LIFO）等数据结构。
• 动态数据存储: 当数据量不确定或需要动态调整时，链表是一个很好的选择。

注意事项

• 内存开销: 链表的每个元素都需要额外的内存来存储前后指针，因此内存开销比数组大。
• 随机访问性能差: 链表不支持随机访问，访问某个元素需要从头或尾开始遍历。

源代码阅读

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.// Package list implements a doubly linked list.
//
// To iterate over a list (where l is a *List):
//
//	for e := l.Front(); e != nil; e = e.Next() {
//		// do something with e.Value
//	}
package list// Element is an element of a linked list.
type Element struct {//双链表元素中的下一个和上一个指针。//为了简化实现，在内部实现了列表l//作为一个环，这样&l.root既是最后一个元素的下一个元素//list元素（l.Back（））和第一个列表的前一个元素//元素（l.Front（））。next, prev *Element// The list to which this element belongs.list *List// The value stored with this element.Value any
}// Next returns the next list element or nil.
func (e *Element) Next() *Element {if p := e.next; e.list != nil && p != &e.list.root {return p}return nil
}// Prev returns the previous list element or nil.
func (e *Element) Prev() *Element {if p := e.prev; e.list != nil && p != &e.list.root {return p}return nil
}// List represents a doubly linked list.
// The zero value for List is an empty list ready to use.
type List struct {root Element // sentinel list element, only &root, root.prev, and root.next are usedlen  int     // current list length excluding (this) sentinel element
}// Init initializes or clears list l.
func (l *List) Init() *List {l.root.next = &l.rootl.root.prev = &l.rootl.len = 0return l
}// New returns an initialized list.
func New() *List { return new(List).Init() }// Len returns the number of elements of list l.
// The complexity is O(1).
func (l *List) Len() int { return l.len }// Front returns the first element of list l or nil if the list is empty.
func (l *List) Front() *Element {if l.len == 0 {return nil}return l.root.next
}// Back returns the last element of list l or nil if the list is empty.
func (l *List) Back() *Element {if l.len == 0 {return nil}return l.root.prev
}// lazyInit lazily initializes a zero List value.
func (l *List) lazyInit() {if l.root.next == nil {l.Init()}
}// insert inserts e after at, increments l.len, and returns e.
func (l *List) insert(e, at *Element) *Element {e.prev = ate.next = at.nexte.prev.next = ee.next.prev = ee.list = ll.len++return e
}// insertValue is a convenience wrapper for insert(&Element{Value: v}, at).
func (l *List) insertValue(v any, at *Element) *Element {return l.insert(&Element{Value: v}, at)
}// remove removes e from its list, decrements l.len
func (l *List) remove(e *Element) {e.prev.next = e.nexte.next.prev = e.preve.next = nil // avoid memory leakse.prev = nil // avoid memory leakse.list = nill.len--
}// move moves e to next to at.
func (l *List) move(e, at *Element) {if e == at {return}e.prev.next = e.nexte.next.prev = e.preve.prev = ate.next = at.nexte.prev.next = ee.next.prev = e
}// Remove removes e from l if e is an element of list l.
// It returns the element value e.Value.
// The element must not be nil.
func (l *List) Remove(e *Element) any {if e.list == l {// if e.list == l, l must have been initialized when e was inserted// in l or l == nil (e is a zero Element) and l.remove will crashl.remove(e)}return e.Value
}// PushFront inserts a new element e with value v at the front of list l and returns e.
func (l *List) PushFront(v any) *Element {l.lazyInit()return l.insertValue(v, &l.root)
}// PushBack inserts a new element e with value v at the back of list l and returns e.
func (l *List) PushBack(v any) *Element {l.lazyInit()return l.insertValue(v, l.root.prev)
}// InsertBefore inserts a new element e with value v immediately before mark and returns e.
// If mark is not an element of l, the list is not modified.
// The mark must not be nil.
func (l *List) InsertBefore(v any, mark *Element) *Element {if mark.list != l {return nil}// see comment in List.Remove about initialization of lreturn l.insertValue(v, mark.prev)
}// InsertAfter inserts a new element e with value v immediately after mark and returns e.
// If mark is not an element of l, the list is not modified.
// The mark must not be nil.
func (l *List) InsertAfter(v any, mark *Element) *Element {if mark.list != l {return nil}// see comment in List.Remove about initialization of lreturn l.insertValue(v, mark)
}// MoveToFront moves element e to the front of list l.
// If e is not an element of l, the list is not modified.
// The element must not be nil.
func (l *List) MoveToFront(e *Element) {if e.list != l || l.root.next == e {return}// see comment in List.Remove about initialization of ll.move(e, &l.root)
}// MoveToBack moves element e to the back of list l.
// If e is not an element of l, the list is not modified.
// The element must not be nil.
func (l *List) MoveToBack(e *Element) {if e.list != l || l.root.prev == e {return}// see comment in List.Remove about initialization of ll.move(e, l.root.prev)
}// MoveBefore moves element e to its new position before mark.
// If e or mark is not an element of l, or e == mark, the list is not modified.
// The element and mark must not be nil.
func (l *List) MoveBefore(e, mark *Element) {if e.list != l || e == mark || mark.list != l {return}l.move(e, mark.prev)
}// MoveAfter moves element e to its new position after mark.
// If e or mark is not an element of l, or e == mark, the list is not modified.
// The element and mark must not be nil.
func (l *List) MoveAfter(e, mark *Element) {if e.list != l || e == mark || mark.list != l {return}l.move(e, mark)
}// PushBackList inserts a copy of another list at the back of list l.
// The lists l and other may be the same. They must not be nil.
func (l *List) PushBackList(other *List) {l.lazyInit()for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {l.insertValue(e.Value, l.root.prev)}
}// PushFrontList inserts a copy of another list at the front of list l.
// The lists l and other may be the same. They must not be nil.
func (l *List) PushFrontList(other *List) {l.lazyInit()for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {l.insertValue(e.Value, &l.root)}
}