Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push
, peek
, pop
, and empty
).
Implement the MyQueue
class:
void push(int x)
Pushes element x to the back of the queue.int pop()
Removes the element from the front of the queue and returns it.int peek()
Returns the element at the front of the queue.boolean empty()
Returnstrue
if the queue is empty,false
otherwise.
Notes:
 You must use only standard operations of a stack, which means only
push to top
,peek/pop from top
,size
, andis empty
operations are valid.  Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (doubleended queue) as long as you use only a stack's standard operations.
Example 1:
Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]
Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false
Constraints:
 1 <= x <= 9
 At most
100
calls will be made topush
,pop
,peek
, andempty
.  All the calls to
pop
andpeek
are valid.
Followup: Can you implement the queue such that each operation is amortized O(1)
time complexity? In other words, performing n operations will take overall O(n)
time even if one of those operations may take longer.

