Leetcode: Implement 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.

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).

class Stack {
public:
queue<int> qfront, qback;

void flush(queue<int> &queue1, queue<int> &queue2) {
int value;

while (queue1.size() > 1) {
value = queue1.front();
queue1.pop();
queue2.push(value);
}
}

// Push element x onto stack.
void push(int x) {
qfront.push(x);
}

// Removes the element on top of the stack.
void pop() {
queue<int> tmp;

if (qfront.size() == 0) {
tmp = qfront;
qfront = qback;
qback = tmp;
}
flush(qfront, qback);
qfront.pop();
}

// Get the top element.
int top() {
queue<int> tmp;
int ret;

if (qfront.size() == 0) {
tmp = qfront;
qfront = qback;
qback = tmp;
}

flush(qfront, qback);
ret = qfront.front();

return ret;
}

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