Leetcode 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).

注意栈是先进后出，队列是先进先出，需要定义两个队列，保证一个为空，一个保存元素，入栈的时候把元素x放到空队列中，然后把另一个队列中的元素出队列并放入到空队列中。

class Stack {
public:
queue<int> p,q;
// Push element x onto stack.
void push(int x) {
if(p.empty())
{
p.push(x);
while(!q.empty())
{
p.push(q.front());
q.pop();
}
}
else if(q.empty())
{
q.push(x);
while(!p.empty())
{
q.push(p.front());
p.pop();
}
}
}

// Removes the element on top of the stack.
void pop() {
if(!p.empty())
{
p.pop();
}
else
q.pop();
}

// Get the top element.
int top() {
if(!p.empty())
return p.front();
else
return q.front();
}

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