Implement Stack using Queues

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.

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题目大意：用队列，实现栈。如果你的语言不支持队列，也可以用列表来模仿实现。那就用ArrayList，一个就够了，用起来简单

代码如下：

class MyStack {        List<Integer> stack = new ArrayList<>();        // Push element x onto stack.    public void push(int x) {        stack.add(x);    }    // Removes the element on top of the stack.    public void pop() {        if(!empty()){            stack.remove(stack.size()-1);        }    }    // Get the top element.    public int top() {        if(!empty()){            return stack.get(stack.size()-1);        }        return -1;    }    // Return whether the stack is empty.    public boolean empty() {        return stack.size()==0;    }}

参考博客：http://www.bubuko.com/infodetail-934509.html

也有像剑指offer上说的用两个队列来实现stack

代码如下：

记得区分queue用add，peek,poll,而stack用push，pop,peek

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

参考博客：http://www.cnblogs.com/easonliu/p/4568492.html