leetcode:用两个栈实现一个队列 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).

class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {

}

/** Push element x to the back of queue. */
void push(int x) {
while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
st1.push(x);
while(!st2.empty())
{
st1.push(st2.top());
st2.pop();
}
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
int top=st1.top();
st1.pop();
return top;
}

/** Get the front element. */
int peek() {
return st1.top();;
}

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

/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* bool param_4 = obj.empty();
*/