[英]How to implement queue with a single stack in C++
In this C++ code I'm implementing Queue
with a Single stack
instance.在此 C++ 代码中,我使用单
stack
实例实现Queue
。 I found this code in GeeksForGeeks.我在 GeeksForGeeks 中找到了这段代码。 Url Here
网址在这里
#include <bits/stdc++.h>
using namespace std;
class Queue
{
private:
stack<int> s;
public:
void enque(int x)
{
s.push(x);
}
int deque()
{
if (s.empty())
{
cout << "Q is empty" << endl;
return -1;
}
int x = s.top();
s.pop();
if (s.empty())
{
return x;
}
// I'm not able to understand these 3 lines after this comment
int item = deque();
s.push(x);
return item;
}
};
int main()
{
Queue q;
q.enque(1);
q.enque(2);
q.enque(3);
cout << "Output: " << q.deque() << endl;
cout << "Output: " << q.deque() << endl;
cout << "Output: " << q.deque() << endl;
cout << "Output: " << q.deque() << endl;
return 0;
}
But I cannot understand these 3 lines但我无法理解这 3 行
int item = deque();
s.push(x);
return item;
Problem
问题
How after calling deque() recursively the compiler reaches the next lines to push x
again to the stack.在递归调用 deque() 之后,编译器如何到达下一行将
x
再次压入堆栈。 And how it is retaining the value of x after recursive function call.以及它如何在递归函数调用后保留 x 的值。
The code isn't really using a single stack, its using the built-in stack as a second stack via the recursive call to deque
.代码并不是真正使用单个堆栈,而是通过递归调用
deque
使用内置堆栈作为第二个堆栈。 The code is equivalent to:代码等价于:
#include <iostream>
#include <stack>
class Queue
{
private:
std::stack<int> s;
public:
void enque(int x)
{
s.push(x);
}
int deque()
{
if (s.empty())
{
std::cout << "Q is empty\n";
return -1;
}
std::stack<int> temp;
while (s.size() != 1)
{
temp.push(s.top());
s.pop();
}
int result = s.top();
s.pop();
while (!temp.empty())
{
s.push(temp.top());
temp.pop();
}
return result;
}
};
int main()
{
Queue q;
q.enque(1);
q.enque(2);
q.enque(3);
std::cout << "Output: " << q.deque() << "\n";
std::cout << "Output: " << q.deque() << "\n";
std::cout << "Output: " << q.deque() << "\n";
std::cout << "Output: " << q.deque() << "\n";
return 0;
}
I can't think of a reason you'd implement a queue this way, as the queue grows in size deque
gets more and more expensive, using the original code you'd ultimately end up with a stack overflow.我想不出你以这种方式实现队列的原因,随着队列大小的增长,
deque
变得越来越昂贵,使用原始代码最终会导致堆栈溢出。 If you need a queue use std::queue
or std::deque
(by default std::queue
is just a wrapper round std::deque
which hides the push_front
/ pop_back
methods)如果您需要队列,请使用
std::queue
或std::deque
(默认情况下, std::queue
只是std::deque
的包装器,它隐藏了push_front
/ pop_back
方法)
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