leetcode 225: Implement Stack using Queues，leetcodequeues

leetcode 225: Implement Stack using Queues，leetcodequeues
Implement Stack using QueuesTotal Accepted: 3468 Total Submissions: 11988

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 onlypush to back,peek/pop from front, size, andis 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]

沒想出啥好的辦法. 就是兩個queues, 輪流把除最後一個之外的都放入另一個queue中.

[CODE]

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