225. Implement Stack using Queues

225. Implement Stack using Queues

Implement 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.

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).

解析：用隊列實現棧，之前做過用兩個棧實現隊列，所以想到用兩個棧來實現隊列，但是看Discuss的時候看到 只用一個隊列就可以。具體是：在新元素入隊後，把這個元素之前的所有的元素都出隊並且重新入隊。這樣就能保證最後插入隊列的元素始終在隊列的最前端，比如插入a,b,c,d這四個元素，隊列中元素的順序依次爲a,ab,abc,abcd，這樣插入的時間複雜度是O(n)，彈出和獲取最後一個元素的時間複雜度是O(1)，是不是很奇妙？圖示如下：

public class MyStack {

    private Queue<Integer> queue;
    
    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<>();
        
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.offer(x);
        for(int i = 0 ; i <queue.size()-1;i++){
            queue.offer(queue.peek());
            queue.poll();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }
    
    /** Get the top element. */
    public int top() {
        
        return queue.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return  queue.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();
 */