LeetCode: 232. Implement Queue using Stacks

题目描述

Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue.
pop() – Removes the element from in front of queue.
peek() – Get the front element.
empty() – Return whether the queue is empty.
Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

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

解题思路

用两个栈来模拟队列。

AC 代码

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        m_stk.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(m_que.empty())
        {
            MoveStk2Que();
        }
        
        int front = m_que.top();
        m_que.pop();
        return front;
    }
    
    /** Get the front element. */
    int peek() {
        if(m_que.empty())
        {
            MoveStk2Que();
        }
        
        return m_que.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return (m_stk.empty() && m_que.empty());
    }
private:
    void MoveStk2Que()
    {
        while(!m_stk.empty())
        {
            m_que.push(m_stk.top());
            m_stk.pop();
        }
    }
private:
    stack<int> m_stk;
    stack<int> m_que;
};

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