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.
Credits: Special thanks to @jianchao.li.fighter for adding this problem and all test cases.
就是两个queues，轮流把除最后一个之外的都放入另一个queue中。
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 = 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 = q1; q1 = q2; q2 = temp; return x; } // Return whether the stack is empty. public boolean empty() { return q1.isEmpty(); } }