LeetCode225 Implemet Stack using Queues Java題解

標籤：stack   queue   implemet stack   leetcode   java   

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 emptyoperations 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).

解答：

用隊列實現棧，隊列是先進先出，棧是先進後出，所以每一個隊列元素進的時候，我們把這個元素後面的所有隊列的元素重新出隊列再進隊列，這樣就保證了最後面進入隊列的元素可以最先出隊列

代碼：

class MyStack {  Queue<Integer> queue=new LinkedList<Integer>();    public void push(int x) {    queue.offer(x);    int i=queue.size()-1;    while(i>0)    {    queue.offer(queue.poll());    i--;    }            }    public void pop() {        queue.poll();    }    public int top() {        return queue.peek();    }       public boolean empty() {    return queue.isEmpty();            }}

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LeetCode225 Implemet Stack using Queues Java題解