LeetCode 232. Implement Queue using Stacks

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).

题意：使用栈实现队列的如下操作：
push(x):将x加入队尾
pop():移除队首元素
peek():取队首元素
empty():判断队列是否为空
Notes：
只能使用栈的基本操作：push， pop ，peek， size， isEmpty

思路：使用两个栈。
元素出队时，先将栈stack1中除栈顶元素外的所有元素出栈，存入stack2中；移除栈顶元素（即队尾元素）后，再将stack2中的元素放入stack1中

class MyQueue {
    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();
    private int top;

    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        if(stack1.isEmpty())//stack1.size() == 0
            top = x;
        stack1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while(stack1.size() > 1){
            top = stack1.pop();
            stack2.push(top);
        }
        int x = stack1.pop();
        while(stack2.size() > 0){
            int i = stack2.pop();
            stack1.push(i);
        }
        return x;
    }
    
    /** Get the front element. */
    public int peek() {
        return top;
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty();
    }
}