Implement Queue using Stacks

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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

  • You must use only standard operations of a stack — which means only push to toppeek/pop from topsize, and is 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).

C# Program

public class MyQueue {
    
    Stack<int> inputStack;
    Stack<int> outputStack;
    /** Initialize your data structure here. */
    public MyQueue() {
        inputStack = new Stack<int>();
        outputStack = new Stack<int>();
    }
    
    /** Push element x to the back of queue. */
    public void Push(int x) {
        inputStack.Push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int Pop() {
        if(outputStack.Count > 0)
            return outputStack.Pop();
        else
        {
            while(inputStack.Count > 0)
                outputStack.Push(inputStack.Pop());
        }
            return outputStack.Pop();
    }
    
    /** Get the front element. */
    public int Peek() {
         if(outputStack.Count > 0)
            return outputStack.Peek();
        else
        {
            while(inputStack.Count > 0)
                outputStack.Push(inputStack.Pop());    
        }
        return outputStack.Peek();
    }
    
    /** Returns whether the queue is empty. */
    public bool Empty() {
        return inputStack.Count == 0 && outputStack.Count == 0;
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.Push(x);
 * int param_2 = obj.Pop();
 * int param_3 = obj.Peek();
 * bool param_4 = obj.Empty();
 */

One thought on “Implement Queue using Stacks

  1. Pingback: 100 Days Leetcode Challenge – Passion of Programming

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