LeetCode刷题EASY篇Implement Stack using Queues

题目

mplement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

十分钟尝试

既然队列是先进先出，stack是先进后出，想到的是反转操作，但是什么时候反转呢？加入元素的时候 ？这样会重复反转，也不对。那只能加入的元素与其他已经存在的元素进行反转操作。

加入一个新元素，把原来队列里面的元素出队，然后重新入队，这样新加入的元素就是队首元素。

class MyStack {

    
    private LinkedList<Integer> list;
    
    /** Initialize your data structure here. */
    public MyStack() {
        list=new LinkedList();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        list.add(x);
        int size=list.size();
        while(size>1){
            list.add(list.remove());
            size--;
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return list.pop();
    }
    
    /** Get the top element. */
    public int top() {
        return list.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return list.isEmpty();
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */