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