leetcode刷題（232）——用棧實作隊列

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).

class MyQueue {
    
    private Stack<Integer> stack1 = new Stack<>();
    private Stack<Integer> stack2 = new Stack<>();
    
    /** Initialize your data structure here. */
    public MyQueue() {
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while(!stack2.isEmpty()){
            stack1.push(stack2.pop());
        }
        stack1.push(x);
        while (!stack1.isEmpty())
        stack2.push(stack1.pop());
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return stack2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return stack2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return (stack1.size()+stack2.size())==0;
    }
}

/**
 * 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();
 */