225. Implement Stack using Queues

题目

Implement the following operations of a stack using queues.

push(x) – Push element x onto stack.
pop() – Removes the element on top of the stack.
top() – Get the top element.
empty() – Return whether the stack is empty.
Notes:
You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

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思路

栈的相关实现

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代码

class Stack {    queue<int> q1;    queue<int> q2;public:    // Push element x onto stack.    void push(int x) {        if(q1.empty())        {            q2.push(x);        }        else        {            q1.push(x);        }    }    // Removes the element on top of the stack.    void pop() {        if(q1.empty())        {            transElement(2);            q2.pop();        }        else        {            transElement(1);            q1.pop();        }    }    // Get the top element.    int top() {        int result;        if(q1.empty())        {            //q2转q1            transElement(2);            result = q2.front();            q1.push(q2.front());            q2.pop();        }        else        {            transElement(1);            result = q1.front();            q2.push(q1.front());            q1.pop();        }        return result;    }    // Return whether the stack is empty.    bool empty() {        return q1.empty() && q2.empty();    }    //1为q1->q2,2为q2->q1    void transElement(int num)    {        if(num == 1)        {            while(q1.size() > 1)            {                q2.push(q1.front());                q1.pop();            }        }        else        {            while(q2.size() > 1)            {                q1.push(q2.front());                q2.pop();            }        }    }};