Leetcode 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).

用栈实现队列。
可以用两个栈实现，也可以像我这样显式地用一个栈实现，其实本质还是两个栈。

关键在于push操作如何让新的元素放入栈底，显然可以用递归的方式。

class MyQueue {
public:
stack<int> s;
/** Initialize your data structure here. */
MyQueue() {

}

/** Push element x to the back of queue. */
void push(int x) {
if(s.empty())
{
s.push(x);
return ;
}
int tmp = pop();
push(x);
s.push(tmp);
}

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

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

/** Returns whether the queue is empty. */
bool empty() {
return s.empty();
}
};

/**
* 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();
*/