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
, andis 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 { Stack<Integer> end = new Stack<>(); Stack<Integer> front = new Stack<>(); // Push element x to the back of queue. public void push(int x) { end.push(x); } // Removes the element from in front of queue. public void pop() { if(front.isEmpty()) { while(!end.isEmpty()) { front.push(end.pop()); } } if(!front.isEmpty()) front.pop(); } // Get the front element. public int peek() { if(front.isEmpty()) { while(!end.isEmpty()) { front.push(end.pop()); } } return front.peek(); } // Return whether the queue is empty. public boolean empty() { return front.isEmpty() && end.isEmpty(); } }