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Implement Stack using Queues

Problem​

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (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.

Notes:

  • You must use only standard operations of a queue, which means that 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.

Solution Approach​

Expected Time complexity: O(n)O(n)

Click - to see solution code
class MyStack {
    deque<int> q;

   public:
    MyStack() {}

    void push(int x) { q.push_back(x); }

    int pop() {
        int x = top();
        q.pop_back();
        return x;
    }

    int top() { return q.back(); }

    bool empty() { return q.empty(); }
};