236 lines
6.4 KiB
Go
236 lines
6.4 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package list implements a doubly linked list.
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//
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// To iterate over a list (where l is a *List):
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// for e := l.Front(); e != nil; e = e.Next() {
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// // do something with e.Value
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// }
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//
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package list
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// Element is an element of a linked list.
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type Element[T any] struct {
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// Next and previous pointers in the doubly-linked list of elements.
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// To simplify the implementation, internally a list l is implemented
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// as a ring, such that &l.root is both the next element of the last
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// list element (l.Back()) and the previous element of the first list
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// element (l.Front()).
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next, prev *Element[T]
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// The list to which this element belongs.
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list *List[T]
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// The value stored with this element.
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Value T
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}
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// Next returns the next list element or nil.
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func (e *Element[T]) Next() *Element[T] {
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if p := e.next; e.list != nil && p != &e.list.root {
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return p
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}
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return nil
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}
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// Prev returns the previous list element or nil.
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func (e *Element[T]) Prev() *Element[T] {
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if p := e.prev; e.list != nil && p != &e.list.root {
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return p
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}
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return nil
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}
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// List represents a doubly linked list.
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// The zero value for List is an empty list ready to use.
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type List[T any] struct {
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root Element[T] // sentinel list element, only &root, root.prev, and root.next are used
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len int // current list length excluding (this) sentinel element
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}
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// Init initializes or clears list l.
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func (l *List[T]) Init() *List[T] {
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l.root.next = &l.root
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l.root.prev = &l.root
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l.len = 0
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return l
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}
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// New returns an initialized list.
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func New[T any]() *List[T] { return new(List[T]).Init() }
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// Len returns the number of elements of list l.
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// The complexity is O(1).
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func (l *List[T]) Len() int { return l.len }
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// Front returns the first element of list l or nil if the list is empty.
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func (l *List[T]) Front() *Element[T] {
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if l.len == 0 {
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return nil
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}
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return l.root.next
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}
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// Back returns the last element of list l or nil if the list is empty.
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func (l *List[T]) Back() *Element[T] {
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if l.len == 0 {
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return nil
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}
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return l.root.prev
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}
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// lazyInit lazily initializes a zero List value.
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func (l *List[T]) lazyInit() {
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if l.root.next == nil {
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l.Init()
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}
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}
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// insert inserts e after at, increments l.len, and returns e.
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func (l *List[T]) insert(e, at *Element[T]) *Element[T] {
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e.prev = at
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e.next = at.next
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e.prev.next = e
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e.next.prev = e
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e.list = l
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l.len++
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return e
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}
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// insertValue is a convenience wrapper for insert(&Element{Value: v}, at).
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func (l *List[T]) insertValue(v T, at *Element[T]) *Element[T] {
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return l.insert(&Element[T]{Value: v}, at)
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}
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// remove removes e from its list, decrements l.len
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func (l *List[T]) remove(e *Element[T]) {
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e.prev.next = e.next
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e.next.prev = e.prev
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e.next = nil // avoid memory leaks
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e.prev = nil // avoid memory leaks
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e.list = nil
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l.len--
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}
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// move moves e to next to at.
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func (l *List[T]) move(e, at *Element[T]) {
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if e == at {
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return
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}
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e.prev.next = e.next
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e.next.prev = e.prev
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e.prev = at
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e.next = at.next
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e.prev.next = e
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e.next.prev = e
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}
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// Remove removes e from l if e is an element of list l.
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// It returns the element value e.Value.
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// The element must not be nil.
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func (l *List[T]) Remove(e *Element[T]) T {
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if e.list == l {
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// if e.list == l, l must have been initialized when e was inserted
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// in l or l == nil (e is a zero Element) and l.remove will crash
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l.remove(e)
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}
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return e.Value
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}
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// PushFront inserts a new element e with value v at the front of list l and returns e.
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func (l *List[T]) PushFront(v T) *Element[T] {
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l.lazyInit()
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return l.insertValue(v, &l.root)
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}
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// PushBack inserts a new element e with value v at the back of list l and returns e.
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func (l *List[T]) PushBack(v T) *Element[T] {
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l.lazyInit()
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return l.insertValue(v, l.root.prev)
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}
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// InsertBefore inserts a new element e with value v immediately before mark and returns e.
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// If mark is not an element of l, the list is not modified.
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// The mark must not be nil.
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func (l *List[T]) InsertBefore(v T, mark *Element[T]) *Element[T] {
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if mark.list != l {
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return nil
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}
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// see comment in List.Remove about initialization of l
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return l.insertValue(v, mark.prev)
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}
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// InsertAfter inserts a new element e with value v immediately after mark and returns e.
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// If mark is not an element of l, the list is not modified.
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// The mark must not be nil.
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func (l *List[T]) InsertAfter(v T, mark *Element[T]) *Element[T] {
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if mark.list != l {
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return nil
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}
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// see comment in List.Remove about initialization of l
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return l.insertValue(v, mark)
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}
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// MoveToFront moves element e to the front of list l.
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// If e is not an element of l, the list is not modified.
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// The element must not be nil.
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func (l *List[T]) MoveToFront(e *Element[T]) {
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if e.list != l || l.root.next == e {
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return
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}
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// see comment in List.Remove about initialization of l
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l.move(e, &l.root)
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}
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// MoveToBack moves element e to the back of list l.
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// If e is not an element of l, the list is not modified.
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// The element must not be nil.
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func (l *List[T]) MoveToBack(e *Element[T]) {
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if e.list != l || l.root.prev == e {
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return
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}
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// see comment in List.Remove about initialization of l
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l.move(e, l.root.prev)
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}
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// MoveBefore moves element e to its new position before mark.
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// If e or mark is not an element of l, or e == mark, the list is not modified.
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// The element and mark must not be nil.
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func (l *List[T]) MoveBefore(e, mark *Element[T]) {
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if e.list != l || e == mark || mark.list != l {
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return
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}
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l.move(e, mark.prev)
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}
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// MoveAfter moves element e to its new position after mark.
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// If e or mark is not an element of l, or e == mark, the list is not modified.
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// The element and mark must not be nil.
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func (l *List[T]) MoveAfter(e, mark *Element[T]) {
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if e.list != l || e == mark || mark.list != l {
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return
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}
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l.move(e, mark)
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}
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// PushBackList inserts a copy of another list at the back of list l.
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// The lists l and other may be the same. They must not be nil.
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func (l *List[T]) PushBackList(other *List[T]) {
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l.lazyInit()
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for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
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l.insertValue(e.Value, l.root.prev)
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}
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}
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// PushFrontList inserts a copy of another list at the front of list l.
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// The lists l and other may be the same. They must not be nil.
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func (l *List[T]) PushFrontList(other *List[T]) {
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l.lazyInit()
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for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
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l.insertValue(e.Value, &l.root)
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}
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}
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