[LeetCode] 225. Implement Stack using Queues Java

標籤：java   eth   param   pen   front   str   example   depend   題意   

題目：

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).

 

題意及分析：用標準的queue實現stack。這裡關鍵就是添加完成時，剛添加的點要在queue最前面，將queue的前面的點依次取出並添加到queue尾就可以實現這個stack添加功能。

代碼： 

public class MyStack {    Queue<Integer> queue ;    /** Initialize your data structure here. */    public MyStack() {        queue = new LinkedList<Integer>();    }        /** Push element x onto stack. */    public void push(int x) {        queue.add(x);        for(int i=0;i<queue.size()-1;i++){        queue.add(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(); */

 

　　

 

[LeetCode] 225. Implement Stack using Queues Java