Implement Queue using Stacks

Problem

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() Returns true 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, and is 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 (double-ended queue) as long as you use only a stack's standard operations.

Solution Approach

Expected Time complexity: $O(n)$

Click - to see solution code

C++

class MyQueue {
    stack<int> q1, q2;

   public:
    MyQueue() {}

    void push(int x) { q2.push(x); }

    int pop() {
        peek();
        int a = q1.top();
        q1.pop();
        return a;
    }

    int peek() {
        if (q1.empty()) {
            while (!q2.empty()) {
                q1.push(q2.top());
                q2.pop();
            }
        }
        return q1.top();
    }

    bool empty() { return (q1.empty() && q2.empty()); }
};