225 Implement Stack using Queues

1 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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

2 尝试解

2.1 分析

用队列实现栈，主要在于删除和查看栈顶元素。只需要一个临时队列，删除时除了最后一个元素全部放入临时队列中，然后返回最后一个元素，令临时队列为原队列。查看栈顶时，则不需要删除。

2.2 代码

class MyStack {
public:
    queue<int> saver;
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        saver.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        queue<int> temp;
        int cur = saver.front();
        saver.pop();
        while(!saver.empty()){
            temp.push(cur);
            cur = saver.front();
            saver.pop();
        }
        saver.swap(temp);
        return cur;
    }
    
    /** Get the top element. */
    int top() {
        queue<int> temp;
        int cur;
        while(!saver.empty()){
            cur = saver.front();
            saver.pop();
            temp.push(cur);
        }
        saver.swap(temp);
        return cur;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return saver.empty();
    }
};

3 标准解

225 Implement Stack using Queues

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