[leetcode] Implement Stack using Queues

题目链接在此。

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).

用2个队列：所有数据放在第一个队列，pop和top操作时要用到第二个队列进行暂时存储，最后第二个队列中的数据还是要按顺序放回第一个队列。

class Stack {
public:
// Push element x onto stack.
void push(int x) {
a.push(x);
}

// Removes the element on top of the stack.
void pop() {
if (empty()) {
cout << "The stack is empty!\n";
return ;
}

while (a.size() != 1) {
int tmp = a.front();
a.pop();
b.push(tmp);
}

a.pop();

while (!b.empty()) {
int tmp = b.front();
b.pop();
a.push(tmp);
}
}

// Get the top element.
int top() {
if (empty()) {
cout << "The stack is empty!\n";
return -1;
}

int head;
while (!a.empty()) {
head = a.front();
a.pop();
b.push(head);
}

while (!b.empty()) {
int tmp = b.front();
b.pop();
a.push(tmp);
}

return head;
}

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

private:
queue<int> a;
queue<int> b;
};