GuilinDev

Lc0232

05 August 2008

232 - Implement Queue using Stacks

原题概述

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).

题意和分析

用栈来实现队列，刚好和225-Implement Stack using Queues相反。这个现实主要是用在读写分离的情况下，函数式编程里面也常实现。

栈和队列的核心不同点就是栈是先进后出，而队列是先进先出，所以要用栈的先进后出的特性来模拟出队列的先进先出。那么怎么做呢，只要我们在插入元素的时候每次都从前面插入即可，比如如果一个队列是1,2,3,4，那么在栈中保存为4,3,2,1，那么返回栈顶元素1，也就是队列的首元素，则问题迎刃而解。所以此题的难度是push函数，我们需要一个辅助栈s2，把s的元素也逆着顺序存入temp中，此时加入新元素x，再把temp中的元素存回来，这样就是我们要的顺序了，其他三个操作也就直接调用栈的操作即可。

代码

把逻辑都写在push里面

class MyQueue {

    Stack<Integer> queue = new Stack<>();

    /** Initialize your data structure here. */
    public MyQueue() {

    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        Stack<Integer> temp = new Stack<Integer>();
        while(!queue.empty()){
            temp.push(queue.pop());
        }
        queue.push(x);
        while(!temp.empty()){
            queue.push(temp.pop());
        }
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return queue.pop();
    }

    /** Get the front element. */
    public int peek() {
        return queue.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return queue.empty();
    }
}

/**
 * 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();
 * boolean param_4 = obj.empty();
 */

上面那个解法简单容易理解，但是效率不高，因为每次在push的时候，都要翻转两边栈，下面这个方法使用了两个栈s1和s2，其中新进栈的都先缓存在s1中，当要pop和peek的时候，才将s1中所有元素移到s2中操作，这样可以得到优化。

class MyQueue {

    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();

    /** Initialize your data structure here. */
    public MyQueue() {

    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek();
        return s2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (s2.isEmpty()) {
            while (!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}

/**
 * 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();
 * boolean param_4 = obj.empty();
 */

直接调用原生stack的方法，遇到push就把元素从s2往s1倒，遇到pop和peek就把元素从s1往s2倒

class MyQueue {

    Stack<Integer> s1;
    Stack<Integer> s2;
    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while (!s2.isEmpty()) {
            s1.push(s2.pop());
        }
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while (!s1.isEmpty()) {
            s2.push(s1.pop());
        }
        return s2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        while(!s1.isEmpty()) {
            s2.push(s1.pop());
        }
        return s2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}

/**
 * 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();
 * boolean param_4 = obj.empty();
 */

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