232. Implement Queue using Stacks

用stack 實現佇列的基本功能，比如入隊(隊尾巴)， 出隊(在隊頭出現佇列)

• s1作輸入棧，逐個元素壓棧，以此模擬佇列元素的入隊。直接入佇列即可
• • 如果s2不為空的話，直接pop()，直接彈出來的情況
  • 如果為空的話，需要將s1中的值進行轉過即可
    ==helper的功能就是將元素從s1移動到s2 ==
    可以將helper程式碼放在peek中
    將peek()和push()操作進行之前都需要進行
    

程式碼

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.

class MyQueue {

    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();
    
    public void push(int x) {
        input.push(x);
    }

    public void pop() {
        peek();
        output.pop();
    }
    
    public int peek() {
        if (output.empty(
 
))
            while (!input.empty())
                output.push(input.pop());
        return output.peek();
    }
    public boolean empty() {
        return input.empty() && output.empty();
    }
}

將helper程式碼抽象抽象出來

class MyQueue {

    /** Initialize your data structure here. */
    private
 
 Stack<Integer> in;
    private Stack<Integer> out;
    public MyQueue() {
        in =new Stack<>();
        out = new Stack<>();
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        in.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
         helper();
       return out.pop();
      }
    
    /** Get the front element. */
    private void helper(){
        if(out.isEmpty()){
            while(!in.isEmpty()){
                out.push(in.pop());
            }
        }
    }
    public int peek() {
        //如果in中有元素的情況
       //處理前面的元素, 如果
             helper();
     return   out.peek();
        
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return in.isEmpty() && out.isEmpty();
    }
}