232. Implement Queue using Stacks

Posted 2022-12-14 Carl_Hugo

https://leetcode.com/problems/implement-queue-using-stacks/description/

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.
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：
我们设定s1是入栈的，s2是出栈的

入队列：直接压入元素至s1即可

出队列：如果s2不为空，把s2中的栈顶元素直接弹出。否则，把s1的所有元素全部弹出压入s2中，再弹出s2的栈顶元素

public class MyQueue 
    Stack<Integer> input = new Stack<Integer>();
    Stack<Integer> output = new Stack<Integer>();
    /** Initialize your data structure here. */
    public MyQueue() 
        input = new Stack<Integer>();
        output = new Stack<Integer>();
    

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

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

    /** Get the front element. */
    public int peek() 
        if(output.isEmpty())
            while(!input.isEmpty())
                output.push(input.pop());
            
        
        return output.peek();
    

    /** Returns whether the queue is empty. */
    public boolean empty() 
        return output.isEmpty()&&input.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();
 */