232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue.

pop() – Removes the element from in front of queue.

peek() – Get the front element.

empty() – Return whether the queue is empty.

Notes:

You must use only standard operations of a stack – which means only push to top, peek/pop from top, size, and is empty operations are valid.

Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.

You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

用st1保存入队的数。

用st2来删除出队的数,此时为保证入队顺序来显示队头，需要逆序，此时st1就为空。

用st2.empty做标记，来记录是否进行过逆序操作。

class Queue {
public:
// Push element x to the back of queue.
void push(int x)
{

while(!st2.empty())
{
st1.push(st2.top());
st2.pop();
}
st1.push(x);

}

// Removes the element from in front of queue.
void pop(void) {

while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
st2.pop();

}

// Get the front element.
int peek(void)
{
while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
return st2.top();
}

// Return whether the queue is empty.
bool empty(void)
{
return st1.empty() && st2.empty();
}
private:
stack<int> st1;
stack<int> st2;
};