LeetCode: 225. Implement Stack using Queues

题目描述

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).

解题思路

用两个队列，将 push 的数据反向写入队列中。

AC 代码

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

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * bool param_4 = obj.empty();
 */