栈和队列的互相实现

两个栈实现队列

两个栈，一个尾端压入，一个头端弹出

• 压入时，都压到一个栈里面
• 只有当弹出栈空了时，才将压入栈里面的元素都转到弹出栈中。

class Solution
{
public:
    void push(int node) {
        stack1.push(node);
    }

    int pop() {
        if(stack2.empty())
        {
            while(!stack1.empty())
            {
                int temp = stack1.top();
                stack2.push(temp);
                stack1.pop();
            }
        }
        if(stack2.empty()) return 0;
        
        int top = stack2.top();
        stack2.pop();
        return top;
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};

两个队列实现栈

实现上 没有两个栈做队列那么优雅。

• 一个队列push，保持只有一个队列有元素
• pop时，将有元素的那个队列除了最后一个元素，都转到另一个队列，把剩余的一个元素pop

#pragma once

#include <queue>
#include <iostream>
using namespace std;

class MyStack
{
public:
	int pop();
	void push(int val);
	friend ostream& operator<<(ostream& os, MyStack& obj);
private:
	queue<int> que1;
	queue<int> que2;

};

inline int MyStack::pop()
{
	int popval = 0;
	queue<int> *pQueValid;
	queue<int> *pQueWait;

	if (que1.empty())
	{
		pQueValid = &que2;
		pQueWait = &que1;
	}
	else
	{
		pQueValid = &que1;
		pQueWait = &que2;
	}
	if (pQueValid->empty()) return 0;
	int size = pQueValid->size();
	for (int i = 0; i < size - 1; ++i)
	{
		pQueWait->push(pQueValid->front());
		pQueValid->pop();
	}
	popval = pQueValid->front();
	pQueValid->pop();
	
	return popval;
}

inline void MyStack::push(int val)
{
	if (que1.empty())
		que2.push(val);
	else
		que1.push(val);
	cout << "push: " << val << endl;
}

ostream & operator<<(ostream & os, MyStack & obj)
{
	os << "que1:";
	queue<int> t1(obj.que1);
	while (!t1.empty())
	{
		os << t1.front();
		t1.pop();
	}
	os << endl;

	os << "que2:";
	queue<int> t2(obj.que2);
	while (!t2.empty())
	{
		os << t2.front();
		t2.pop();
	}
	os << endl;
	return os;
}

测试

#include "MyStack.h"
#include <iostream>
using namespace std;
int main()
{
	MyStack stack;
	stack.push(1);
	stack.push(2);
	stack.push(3);
	stack.push(4);
	//cout << stack << endl;
	cout << stack.pop() << endl;
	cout << stack.pop() << endl;

	stack.push(6);

	cout << stack.pop() << endl;
	cout << stack.pop() << endl;
	cout << stack.pop() << endl;
	cout << stack.pop() << endl;

	stack.push(2);
	stack.push(3);

	cout << stack.pop() << endl;
	cout << stack.pop() << endl;
	cout << stack.pop() << endl;
	cout << stack.pop() << endl;

	system("pause");
	return 0;
}