[leetcode 234]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)。
使用两个栈，模拟队列操作
AC代码

class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
s1.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if(s2.empty())
{
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
}
s2.pop();
}

// Get the front element.
int peek(void) {
if(s2.empty())
{
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
}
return s2.top();
}

// Return whether the queue is empty.
bool empty(void) {
return s1.empty()&&s2.empty();
}
private :
stack<int> s1;
stack<int> s2;
};

其他Leetcode题目AC代码：https://github.com/PoughER/leetcode