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 may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
• 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 – which means only push to back, pop from front, size, and is empty operations are valid.

``````// Runtime: 0 ms 临时理解为计时故障
class Stack {
public:
// Push element x onto stack.
void push(int x) {
gather(q1, q2);
q1.push(x);
}

// Removes the element on top of the stack.
void pop() {
gather(q1, q2);
while (q1.size() > 0 && q1.size() != 1)
{
q2.push(q1.front());
q1.pop();
}
q1.pop();
}

// Get the top element.
int top() {
gather(q1, q2);
while (q1.size() > 0 && q1.size() != 1)
{
q2.push(q1.front());
q1.pop();
}
int n = q1.front();
q2.push(q1.front());
q1.pop();

return n;
}

// Return whether the stack is empty.
bool empty() {
return q1.empty() && q2.empty();
}
private:
queue<int> q1;
queue<int> q2;
void gather(queue<int>& q1, queue<int>& q2)
{
while (!q2.empty())
{
q1.push(q2.front());
q2.pop();
}
}
};``````

``````// Runtime: 0 ms
class Stack {
public:
// Push element x onto stack.
void push(int x) {
toQ1(q1, q2);
q1.push(x);
}

// Removes the element on top of the stack.
void pop() {
toQ1(q1, q2);
while (q1.size() > 0 && q1.size() != 1)
{
q2.push(q1.front());
q1.pop();
}
q1.pop();
}

// Get the top element.
int top() {
toQ1(q1, q2);
while (q1.size() > 0 && q1.size() != 1)
{
q2.push(q1.front());
q1.pop();
}
int n = q1.front();
q2.push(q1.front());
q1.pop();

return n;
}

// Return whether the stack is empty.
bool empty() {
return q1.empty() && q2.empty();
}
private:
queue<int> q1;
queue<int> q2;
void toQ1(queue<int>& q1, queue<int>& q2)
{
if (q1.empty())
{
q1 = q2;
queue<int> q;
q2 = q;
}
}
};``````