LeetCode_Implement Stack using Queues

思路：使用双端队deque列实现栈
class Stack {
public:
deque<int> que;
// Push element x onto stack.
void push(int x) {
que.push_front(x);
}

// Removes the element on top of the stack.
void pop() {
que.pop_front();
}

// Get the top element.
int top() {
if(!que.empty())
{
int x = que.front();
return x;
}
}

// Return whether the stack is empty.
bool empty() {
return que.empty();
}
};