225. Implement Stack using Queues (用队列模拟栈)

225. Implement Stack using Queues

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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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class Stack {
public:
// Push element x onto stack.
void push(int x) {
if(q1.empty())
{
q1.push(x);
while(!q2.empty())
{
q1.push(q2.front());
q2.pop();
}
}
else
{
q2.push(x);
while(!q1.empty())
{
q2.push(q1.front());
q1.pop();
}
}
}

// Removes the element on top of the stack.
void pop() {
if(q1.empty()) q2.pop();
else q1.pop();

}

// Get the top element.
int top() {
if(q1.empty()) return q2.front();
else return q1.front();
}

// Return whether the stack is empty.
bool empty() {
return q1.empty()&&q2.empty();
}

private:
queue<int>q1,q2;
};