Tutorialspoint
Problem
Solution
Submissions
Back

Queue Using Two Stacks

Certification: Intermediate Level Accuracy: 0% Submissions: 1 Points: 10

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).

Example 1
    
  
  • Input: ["MyQueue", "push", "push", "peek", "pop", "empty"]

    [[], [1], [2], [], [], []]
    • 
  
  • Output: [null, null, null, 1, 1, false]
    • 
  
  • Explanation: 
    
      
      
    • MyQueue myQueue = new MyQueue();
      • 
      
    • myQueue.push(1); // queue is: [1]
      • 
      
    • myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
      • 
      
    • myQueue.peek(); // return 1
      • 
      
    • myQueue.pop(); // return 1, queue is [2]
      • 
      
    • myQueue.empty(); // return false
      • 
    
    
  
    •
Example 2
    
  
  • Input: ["MyQueue", "push", "push", "push", "pop", "push", "pop", "pop", "pop", "empty"]

    [[], [1], [2], [3], [], [4], [], [], [], []]
    • 
  
  • Output: [null, null, null, null, 1, null, 2, 3, 4, true]
    • 
  
  • Explanation: 
    
      
      
    • MyQueue myQueue = new MyQueue();
      • 
      
    • myQueue.push(1); // queue is: [1]
      • 
      
    • myQueue.push(2); // queue is: [1, 2]
      • 
      
    • myQueue.push(3); // queue is: [1, 2, 3]
      • 
      
    • myQueue.pop(); // return 1, queue is [2, 3]
      • 
      
    • myQueue.push(4); // queue is: [2, 3, 4]
      • 
      
    • myQueue.pop(); // return 2, queue is [3, 4]
      • 
      
    • myQueue.pop(); // return 3, queue is [4]
      • 
      
    • myQueue.pop(); // return 4, queue is []
      • 
      
    • myQueue.empty(); // return true, queue is empty
      • 
    
    
  
    •
Constraints
    
  
  • 1 ≤ x ≤ 9
    • 
  
  • At most 100 calls will be made to push, pop, peek, and empty
    • 
  
  • All the calls to pop and peek are valid
    • 
  
  • Time Complexity: O(1) amortized for all operations
    • 
  
  • Space Complexity: O(n) where n is the number of elements in the queue
    •
QueueStackMicrosoftFacebook
Editorial

Login to view the detailed solution and explanation for this problem.

My Submissions
All Solutions
Refresh
Lang Status Date Code
You do not have any submissions for this problem.
Refresh
User Lang Status Date Code
No submissions found.

Please Login to continue
Solve Problems

 
 
 
Output Window
Clear All Apply Filters

Trending

Others

SIGN IN

Don't have an account? Register

Hint
Solution

Solution Hints

    
  
  • Use one stack for pushing elements and another stack for popping elements
    • 
  
  • When performing a pop or peek operation, if the "pop stack" is empty, transfer all elements from the "push stack" to the "pop stack"
    • 
  
  • This way, the elements are reversed and follow the FIFO order
    • 
  
  • Elements are only moved from the push stack to the pop stack when necessary, making the amortized time complexity O(1) for all operations
    • 
  
  • Remember that a stack is Last-In-First-Out (LIFO), but a queue is First-In-First-Out (FIFO)
    •

Steps to solve by this approach:

 Step 1: Create two stacks - stackPush (for push operations) and stackPop (for pop and peek operations).
 Step 2: For push operation, simply push the element onto stackPush.
 Step 3: For pop and peek operations, check if stackPop is empty.
 Step 4: If stackPop is empty, transfer all elements from stackPush to stackPop, which reverses their order.
 Step 5: This reversal makes the earliest pushed element to be on top of stackPop, which is what we want for FIFO behavior.
 Step 6: For the empty operation, return true if both stacks are empty.
 Step 7: By only transferring elements when necessary, we achieve O(1) amortized time complexity.

Submitted Code :