我可以不使用堆栈而反转队列吗? [英] Can I reverse a queue without using stack?
问题描述
我有一个带有一些数字的队列,例如5、6、7、8在队列中,5在q->前,8在q->后。我可以在队列中反转它们而不使用堆栈吗? / p>
当然!
使用堆栈:O(N)时间。 O(1)内存。
使用关联数组:O(N)时间。 O(1)内存。
使用固定大小的数组:O(N)时间。 O(N)内存。
使用可扩展数组:O(N)时间。 O(N)内存。
使用队列:O(N ^ 2)次。 O(1)内存。
使用关联数组将比使用堆栈花费更多的时间,但是两者的性能和内存使用量
以下片段展示了如何使用这些数据结构。
堆栈:
#O (N)时间。 O(1)内存。
sub reverse_queue_using_stack {
my($ in_q)= @_;
my $ stack = Stack-> new();
while(my($ item)= $ in_q-> dequeue()){
$ stack-> push($ item);
}
my $ out_q =队列-> new();
while(my($ item)= $ stack-> pop()){
$ out_q-> enqueue($ item);
}
return $ out_q;
}
关联数组:
< pre class = lang-perl prettyprint-override>
#O(N)时间。 O(1)内存。
sub reverse_queue_using_dict {
my($ in_q)= @_;
my $ dict =字典-> new();
我的$ i = 0;
while(my($ item)= $ in_q-> dequeue()){
$ dict-> set($ i ++,$ item);
}
my $ out_q =队列-> new();
而($ i--){
$ out_q->入队($ dict-> delete($ i));
}
return $ out_q;
}
固定大小的数组(如果无法获取大小,则排队
#O(N)时间。 O(N)内存。
sub reverse_queue_using_array {
my($ in_q)= @_;
我的$ count = 0;
my $ queue =队列-> new();
while(my($ item)= $ in_q-&dequeue()){
++ $ count;
$ queue-> enqueue($ item);
}
my $ array = Array-> new($ count);我的$ i的
(0 .. $ count-1){
$ array-> set($ i,$ queue-> dequeue());
}
my $ out_q =队列-> new();
for(1 .. $ count){
my $ i = $ count-$ _;
$ out_q-&n; enqueue($ array-> get($ i));
}
return $ out_q;
}
可扩展数组:
< pre class = lang-perl prettyprint-override>
#O(N)时间。 O(N)内存。
sub reverse_queue_using_list {
my($ in_q)= @_;
我的$ list = List-> new();
而(my($ item)= $ in_q-> dequeue()){
$ list-> append($ item);
}
my $ count = $ list-> size();
my $ out_q =队列-> new();
for(1 .. $ count){
my $ i = $ count-$ _;
$ out_q-&n;排队($ list-> get($ i));
}
return $ out_q;
}
队列:
#O(N ^ 2)时间。 O(1)内存。
sub reverse_queue_using_queue {
my($ in_q)= @_;
my $ queue =队列-> new();
my $ out_q =队列-> new();
而(1){
我的($ tail)= $ in_q-> dequeue()
或最后一个;
而(my($ item)= $ in_q-> dequeue()){
$ queue-> enqueue($ tail);
$ tail = $ item;
}
$ out_q->排队($ tail);
($ in_q,$ queue)=($ queue,$ in_q);
}
return $ out_q;
}
使用以下工具进行了测试:
#!/ usr / bin / perl
使用strict;
使用警告;
使用功能qw(say);
#这些的执行无关紧要;
#如果仅使用这些类提供的方法可以解决问题,则
#使用这些类的任何实现都可以解决问题。
{
package Queue;
sub new {my $ class = shift; bless([],$ class)}
子队列{my $ self = shift;推@ $ self,@_; }
子出队{my $ self = shift; @ $ self? shift(@ $ self):()}
}
{
package Stack;
sub new {my $ class = shift; bless([],$ class)}
sub push {my $ self = shift;推@ $ self,@_; }
sub pop {my $ self = shift; @ $ self? pop(@ $ self):()}
}
{
package Array; #固定大小的数组。
使用鲤鱼qw(croak);
sub new {my($ class,$ size)= @_; bless([(undef)x $ size],$ class)}
子大小{my $ self = shift; 0 + @ $ self}
sub get {my($ self,$ i)= @_;嘶哑的!如果$ i< 0 || $ i> = @ $ self; $ self-> [$ i]}
子集{my($ self,$ i,$ item)= @_;嘶哑的!如果$ i< 0 || $ i> = @ $ self; $ self-> [$ i] = $ item; }
}
{
包裹清单; #可扩展数组。
使用鲤鱼qw(croak);
sub new {my $ class = shift; bless([],$ class)}
sub size {my $ self = shift; 0 + @ $ self}
sub get {my($ self,$ i)= @_;嘶哑的!如果$ i <0; $ self-> [$ i]}
子集{my($ self,$ i,$ item)= @_;嘶哑的!如果$ i <0; $ self-> [$ i] = $ item; }
sub append {my($ self,$ item)= @_;推送@ $ self,$ item; }
}
{
package字典;
使用鲤鱼qw(croak);
sub new {my $ class = shift; bless({},$ class)}
sub get {my($ self,$ k)= @_;嘶哑的!如果!exists($ self-> {$ k}); $ self-> {$ k}}
子集{my($ self,$ k,$ item)= @_; $ self-> {$ k} = $ item; }
sub存在{my($ self,$ k)= @_;存在($ self-> {$ k})}
sub delete {my($ self,$ k)= @_;嘶哑的!如果!exists($ self-> {$ k}); delete($ self-> {$ k})}
}
sub purge_queue {
my($ q)= @_;
我的@vals;
而(my($ item)= $ q-> dequeue()){
push @vals,$ item;
}
return @vals;
}
#...
for我的$ reverse_func_name(qw(
reverse_queue_using_stack
reverse_queue_using_dict
reverse_queue_using_array
reverse_queue_using_list
reverse_queue_using_queue
)){
my $ reverse_func = \& $ reverse_func_name;
my $ in_q =队列-> new();
$ in_q->排队($ _)表示‘a’..’j’;
我的$ out_q = $ reverse_func->($ in_q);
说sprintf%-26s%s, $ reverse_func_name:,加入’,purge_queue($ out_q);
}
I have a queue with some numbers for example 5,6,7,8 is in queue, 5 is q->front and 8 is q->rear.. Can I reverse them in queue but without using stacks?
Of course! But it won't be as efficient.
Using a stack: O(N) time. O(1) memory.
Using an associative array: O(N) time. O(1) memory.
Using a fixed-size array: O(N) time. O(N) memory.
Using an extendable array: O(N) time. O(N) memory.
Using a queue: O(N^2) time. O(1) memory.
Using an associative array will use more time than using a stack, but the performance and memory usage of both will scale identically.
The following snippets show how each of those data structures can be used.
Stack:
# O(N) time. O(1) memory.
sub reverse_queue_using_stack {
my ($in_q) = @_;
my $stack = Stack->new();
while ( my ($item) = $in_q->dequeue() ) {
$stack->push($item);
}
my $out_q = Queue->new();
while ( my ($item) = $stack->pop() ) {
$out_q->enqueue($item);
}
return $out_q;
}
Associative array:
# O(N) time. O(1) memory.
sub reverse_queue_using_dict {
my ($in_q) = @_;
my $dict = Dictionary->new();
my $i = 0;
while ( my ($item) = $in_q->dequeue() ) {
$dict->set($i++, $item);
}
my $out_q = Queue->new();
while ($i--) {
$out_q->enqueue($dict->delete($i));
}
return $out_q;
}
Fixed-size array (and a queue if you can't get the size of a queue):
# O(N) time. O(N) memory.
sub reverse_queue_using_array {
my ($in_q) = @_;
my $count = 0;
my $queue = Queue->new();
while ( my ($item) = $in_q->dequeue() ) {
++$count;
$queue->enqueue($item);
}
my $array = Array->new($count);
for my $i (0..$count-1) {
$array->set($i, $queue->dequeue());
}
my $out_q = Queue->new();
for (1..$count) {
my $i = $count - $_;
$out_q->enqueue($array->get($i));
}
return $out_q;
}
Extendable array:
# O(N) time. O(N) memory.
sub reverse_queue_using_list {
my ($in_q) = @_;
my $list = List->new();
while ( my ($item) = $in_q->dequeue() ) {
$list->append($item);
}
my $count = $list->size();
my $out_q = Queue->new();
for (1..$count) {
my $i = $count - $_;
$out_q->enqueue($list->get($i));
}
return $out_q;
}
Queue:
# O(N^2) time. O(1) memory.
sub reverse_queue_using_queue {
my ($in_q) = @_;
my $queue = Queue->new();
my $out_q = Queue->new();
while (1) {
my ($tail) = $in_q->dequeue()
or last;
while ( my ($item) = $in_q->dequeue() ) {
$queue->enqueue($tail);
$tail = $item;
}
$out_q->enqueue($tail);
($in_q, $queue) = ($queue, $in_q);
}
return $out_q;
}
Tested using the following harness:
#!/usr/bin/perl
use strict;
use warnings;
use feature qw( say );
# The implementation of these don't matter;
# if the problem can be solved using only the methods provided by these classes,
# the problem can be solved using any implementation of these classes.
{
package Queue;
sub new { my $class = shift; bless([], $class) }
sub enqueue { my $self = shift; push @$self, @_; }
sub dequeue { my $self = shift; @$self ? shift(@$self) : () }
}
{
package Stack;
sub new { my $class = shift; bless([], $class) }
sub push { my $self = shift; push @$self, @_; }
sub pop { my $self = shift; @$self ? pop(@$self) : () }
}
{
package Array; # Fixed-size array.
use Carp qw( croak );
sub new { my ($class, $size) = @_; bless([ (undef) x $size ], $class) }
sub size { my $self = shift; 0+@$self }
sub get { my ($self, $i) = @_; croak "!" if $i<0 || $i>=@$self; $self->[$i] }
sub set { my ($self, $i, $item) = @_; croak "!" if $i<0 || $i>=@$self; $self->[$i] = $item; }
}
{
package List; # Extendable array.
use Carp qw( croak );
sub new { my $class = shift; bless([], $class) }
sub size { my $self = shift; 0+@$self }
sub get { my ($self, $i) = @_; croak "!" if $i<0; $self->[$i] }
sub set { my ($self, $i, $item) = @_; croak "!" if $i<0; $self->[$i] = $item; }
sub append { my ($self, $item) = @_; push @$self, $item; }
}
{
package Dictionary;
use Carp qw( croak );
sub new { my $class = shift; bless({}, $class) }
sub get { my ($self, $k) = @_; croak "!" if !exists($self->{$k}); $self->{$k} }
sub set { my ($self, $k, $item) = @_; $self->{$k} = $item; }
sub exists { my ($self, $k) = @_; exists($self->{$k}) }
sub delete { my ($self, $k) = @_; croak "!" if !exists($self->{$k}); delete($self->{$k}) }
}
sub purge_queue {
my ($q) = @_;
my @vals;
while ( my ($item) = $q->dequeue() ) {
push @vals, $item;
}
return @vals;
}
# ...
for my $reverse_func_name (qw(
reverse_queue_using_stack
reverse_queue_using_dict
reverse_queue_using_array
reverse_queue_using_list
reverse_queue_using_queue
)) {
my $reverse_func = \&$reverse_func_name;
my $in_q = Queue->new();
$in_q->enqueue($_) for 'a'..'j';
my $out_q = $reverse_func->($in_q);
say sprintf "%-26s %s", "$reverse_func_name:", join ' ', purge_queue($out_q);
}
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