Leetcode：225. Implement Stack using Queues（JAVA）

【问题描述】

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).

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

【思路】

使用两个队列实现stack操作。

【code】

class MyStack {Queue<Integer> q1 = new LinkedList<Integer>();Queue<Integer> q2 = new LinkedList<Integer>();// Push element x onto stack.public void push(int x) {q1.offer(x);}// Removes the element on top of the stack.public void pop() {while (q1.size() > 1) {q2.offer(q1.poll());}q1.poll();Queue<Integer> temp;temp = q1;q1 = q2;q2 = temp;}// Get the top element.public int top() {while (q1.size() > 1) {q2.offer(q1.poll());}int x = q1.poll();q2.offer(x);Queue<Integer> temp;temp = q1;q1 = q2;q2 = temp;return x;}// Return whether the stack is empty.public boolean empty() {return q1.isEmpty();}}