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Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).

Implement the MyStack class:

  • void push(int x) Pushes element x to the top of the stack.
  • int pop() Removes the element on the top of the stack and returns it.
  • int top() Returns the element on the top of the stack.
  • boolean empty() Returns true if the stack is empty, false otherwise.


  • You must use only standard operations of a queue, which means only push to back, peek/pop from front, size, and is empty operations are valid.
  • Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue's standard operations.


Example 1:

["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
[null, null, null, 2, 2, false]

MyStack myStack = new MyStack();
myStack.push(2);; // return 2
myStack.pop(); // return 2
myStack.empty(); // return False



  • 1 <= x <= 9
  • At most 100 calls will be made to push, pop, top, and empty.
  • All the calls to pop and top are valid.


Follow-up: Can you implement the stack using only one queue?

class MyStack { /** Initialize your data structure here. */ public MyStack() { } /** Push element x onto stack. */ public void push(int x) { } /** Removes the element on top of the stack and returns that element. */ public int pop() { } /** Get the top element. */ public int top() { } /** Returns whether the stack is empty. */ public boolean empty() { } } /** * Your MyStack object will be instantiated and called as such: * MyStack obj = new MyStack(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 =; * boolean param_4 = obj.empty(); */