description: 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).
使用queue結構進行實現stack結構，需要處理最后面的一個元素。使用該元素時，要注意兩個queue的相互交換等一些問題的處理技巧。
public class MyStack { Queue<Integer> queue1; Queue<Integer> queue2; /** Initialize your data structure here. */ public MyStack() { queue1 = new LinkedList<Integer>(); queue2 = new LinkedList<Integer>(); } PRivate void p2p() { int len = queue1.size() - 1; for (int i = 0; i < len; i++) { queue2.offer(queue1.poll()); } } private void swapQueue() { Queue temp = queue1; queue1 = queue2; queue2 = temp; } /** Push element x onto stack. */ public void push(int x) { queue1.offer(x); } /** Removes the element on top of the stack and returns that element. */ public int pop() { p2p(); int temp = queue1.poll(); swapQueue(); return temp; } /** Get the top element. */ public int top() { p2p(); int temp = queue1.poll(); swapQueue(); queue1.offer(temp); return temp; } /** Returns whether the stack is empty. */ public boolean empty() { return queue1.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(); */新聞熱點
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