leetcode 232. Implement Queue using Stacks

題目

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

1，元素都是插入s1中；
2，讀取的時候，從s2中讀取，若s2中沒有元素，則將s1中元素取出放入s2中，然後從s2中讀取元素。

class MyQueue {
    Stack<Integer> s1=new Stack<Integer>();
    Stack<Integer> s2=new Stack<Integer>();

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

    }

    // // Removes the element from in front of queue.
    // public void pop() {
    //  if(!s2.isEmpty()){
    //      s2.pop();
    //  }else{
    //      for(int i=0;i<s1.size();i++){//size是會變化的，不能這麼寫
    //          s2.push(s1.pop());
    //      }
    //      s2.pop();
    //  }

    // }
     // Removes the element from in front of queue.  
    public void pop() {  
        if(!s2.isEmpty()) s2.pop();  
        else {  
            while(!s1.isEmpty()) s2.push(s1.pop());  
            s2.pop();  
        }  
    }  

    // // Get the front element.
    // public int peek() {
    //  if(!s2.isEmpty()){
    //      return s2.peek();
    //  }else{
    //      for(int i=0;i<s1.size();i++){//size是會變化的，不能這麼寫
    //          s2.push(s1.pop());
    //      }
    //      return s2.peek();
    //  }

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

    // Return whether the queue is empty.
    public boolean empty() {

        if(s1.isEmpty() && s2.isEmpty())
            return true;
        else
            return false;
    }



}