[LeetCode] 232. Implement Queue using Stacks

問題描述

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).

解題思維

首先我們要先釐清柱列(Queue)與 Stack(堆)的特性，前者為先進先出，後者為後進先出，題目是要我們使用堆來實現柱列的功能，我們需要用兩個堆來完成，
一個存放資料，另一個暫存資料，解題主要思維是利用堆後進先出的特性，將儲存在第一個堆的資料，依序 pop 出來，同時 push 到暫存的堆裡，將要放入
的資料放入第一堆，最後將暫存的堆裡依序吐出到第一堆，剛好最先進去的資料排在第一個。

Enqueue

/**
 * 實作Enqueue功能
 * @param x 資料
 * @remark
 */
public void push(int x) {
        //依序先將存在第一個Stack吐到第二個裡
        while(!stack1.empty()){
            stack2.push(stack1.pop());
        }

        //將我們儲存的元素存到第一個Stack裡
        stack1.push(x);

        //依序將存在暫存Stack的元素依序吐回第一個Stack
        //維持先進去的元素排在最前面
        while(!stack2.empty()){
            stack1.push(stack2.pop());
        }
    }

Dequeue

/**
 * 實作Dequeue功能
 * @return 返回Stack第一個元素
 * @remark
 */
public int pop() {
    //直接pop第一個Stack第一個元素
    return stack1.pop();
}

程式完整解題

/**
Runtime: 0 ms, faster than 100.00% of Java online submissions for Implement Queue using Stacks.
Memory Usage: 40.9 MB, less than 6.25% of Java online submissions for Implement Queue using Stacks.
*/
import java.util.Stack;

class MyQueue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        while(!stack1.empty()){
            stack2.push(stack1.pop());
        }

        stack1.push(x);

        while(!stack2.empty()){
            stack1.push(stack2.pop());
        }
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return stack1.pop();
    }

    /** Get the front element. */
    public int peek() {
        return stack1.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.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();
 */