# Stack using two queues algorithm

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It is named stack as it behaves like a real-world stack, for example — a deck of cards or a pile of plates, etc. A real-world stack allows operations at one end only.

For example, we can place or remove a card or plate from the top of the stack only. Likewise, Stack ADT allows all data operations at one end only. At any given time, we can only access the top element of a stack. This feature makes it LIFO data structure. LIFO stands for Last-in-first-out. Here, the element which is placed inserted or added last, is accessed first.

Stack can either be a fixed size one or it may have a sense of dynamic resizing. Here, we are going to implement stack using arrays, which makes it a fixed size stack implementation.

Stack operations may involve initializing the stack, using it and then de-initializing it. To use a stack efficiently, we need to check the status of stack as well. At all times, we maintain a pointer to the last PUSHed data on the stack. As this pointer always represents the top of the stack, hence named top.

The top pointer provides top value of the stack without actually removing it. Implementation of isempty function in C programming language is slightly different. We initialize top at -1, as the index in array starts from 0.

So we check if the top is below zero or -1 to determine if the stack is empty. The process of putting a new data element onto stack is known as a Push Operation. If the linked list is used to implement the stack, then in step 3, we need to allocate space dynamically.

Accessing the content while removing it from the stack, is known as a Pop Operation. In an array implementation of pop operation, the data element is not actually removed, instead top is decremented to a lower position in the stack to point to the next value. But in linked-list implementation, pop actually removes data element and deallocates memory space. For a complete stack program in C programming language, please click here. Data Structure and Algorithms - Stack Advertisements.

Previous Page. Next Page. Previous Page Print Page.We are given queue data structure, the task is to implement stack using only given queue data structure.

We have discussed a solution that uses two queues. In this article, a new solution is discussed that uses only one queue. This solution assumes that we can find size of queue at any point. The idea is to keep newly inserted element always at rear of queue, keeping order of previous elements same.

Below are complete steps. This article is contributed by Manu Agrawal. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute geeksforgeeks. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Writing code in comment? Please use ide. Stack s.

Curriculum banca e mercatiEnqueue val. Dequeue. Enqueue x. WriteLine "No elements". Peek. Check if a queue can be sorted into another queue using a stack Stack and Queue in Python using queue Module How to efficiently implement k stacks in a single array? How to efficiently implement k Queues in a single array? Load Comments. LinkedList; import java.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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A similar question was asked earlier therebut the question here is the reverse of it, using two queues as a stack. The question Given two queues with their standard operations enqueuedequeueisemptysizeimplement a stack with its standard operations poppushisemptysize.

I am interested in the algorithm more than any specific language implementations. However, I welcome solutions expressed in languages which I am familiar javacpythonvbjavascriptphp. The easiest and maybe only way of doing this is by pushing new elements into the empty queue, and then dequeuing the other and enqeuing into the previously empty queue.

With this way the latest is always at the front of the queue. This would be version B, for version A you just reverse the process by dequeuing the elements into the second queue except for the last one.

Can we just use one queue to implement a stack? I can use two queues, but considering single queue would be more efficient. Here is the code:. Here's my answer - where the 'pop' is inefficient. Seems that all algorithms that come immediately to mind have N complexity, where N is the size of the list: whether you choose to do work on the 'pop' or do work on the 'push'.

The algorithm where lists are traded back and fourth may be better, as a size calculation is not needed, although you still need to loop and compare with empty. Here is my solution that works for O 1 in average case. There are two queues: in and out.

See pseudocode bellow:. As has been mentioned, wouldn't a single queue do the trick? It's probably less practical but is a bit slicker. Pop Operation: Transfer n-1 elements to other queue and delete last from queue for performing pop operation.

Time Complexity: Running Time of pop Operation is O n as each time pop is called, we are transferring all the elements from one queue to oter. So in the above class named "CustomStack" what I am doing is just checking the queue for emptyif empty then insert one and from then on wards insert and then remove insert. By this logic first will come last. Example : In queue I inserted 1 and now trying to insert 2. Second time remove 1 and again insert so it becomes in reverse order.

Pop operation - Ensure that queue q2 is not empty. If it is empty, then dequeue all the elements from q1 except the last element and enqueue it to q2 one by one. Dequeue the last element from q1 and store it as the popped element.

Swap the queues q1 and q2. Return the stored popped element. Peek operation - Ensure that queue q2 is not empty. Dequeue the last element from q1 and store it as the peeked element. Enqueue it back to queue q2 and swap the queues q1 and q2. Return the stored peeked element.Stack is LIFO last in - first out data structure, in which elements are added and removed from the same end, called top. In general stack is implemented using array or linked list, but in the current article we will review a different approach for implementing stack using queues.

In contrast queue is FIFO first in - first out data structure, in which elements are added only from the one side - rear and removed from the other - front. In order to implement stack using queues, we need to maintain two queues q1 and q2. Also we will keep top stack element in a constant memory. The new element is always added to the rear of queue q1 and it is kept as top stack element. Time complexity : O 1 O 1 O 1.

Queue is implemented as linked list and add operation has O 1 O 1 O 1 time complexity. Space complexity : O 1 O 1 O 1. We need to remove the element from the top of the stack. This is the last inserted element in q1. Because queue is FIFO first in - first out data structure, the last inserted element could be removed only after all elements, except it, have been removed. For this reason we need to maintain additional queue q2which will serve as a temporary storage to enqueue the removed elements from q1.

The last inserted element in q2 is kept as top. Then the algorithm removes the last element in q1.

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We swap q1 with q2 to avoid copying all elements from q2 to q1. The algorithm inserts each new element to queue q2 and keep it as the top element. In case queue q1 is not empty there are elements in the stackwe remove all elements from q1 and add them to q2. In this way the new inserted element top element in the stack will be always positioned at the front of q2. Time complexity : O n O n O n. The operations add and remove in linked lists has O 1 O 1 O 1 complexity. The algorithm dequeues an element from queue q1 and keeps front element of q1 as top.

In both approaches empty and top operations have the same implementation.

Circolare osasco n° 154 – convocazione docenti esami integrativiQueue q1 always contains all stack elements, so the algorithm checks q1 size to return if the stack is empty. The top element is kept in constant memory and is modified each time when we push or pop an element. The top element has been calculated in advance and only returned in top operation.

The mentioned above two approaches have one weakness, they use two queues. This could be optimized as we use only one queue, instead of two. When we push an element into a queue, it will be stored at back of the queue due to queue's properties. But we need to implement a stack, where last inserted element should be in the front of the queue, not at the back. To achieve this we can invert the order of queue elements when pushing a new element. The last inserted element is always stored at the front of q1 and we can pop it for constant time.

Queue q1 contains all stack elements, so the algorithm checks if q1 is empty. The top element is always positioned at the front of q1. Algorithm return it. For Approach 1, the pop method returns void when it should return the top.Implement a Queue using 2 stacks s1 and s2.

Then T test cases follow.

**Implement A Queue Using Stacks - The Queue ADT ("Implement Queue Using Stacks" on LeetCode)**

Your Task: Since this is a function problem, you don't need to take inputs. Just complete the provided functions. The printing is done automatically by the driver code. The task is to complete the function specified, and not to write the full code.

If you have purchased any course from GeeksforGeeks then please ask your doubt on course discussion forum. You will get quick replies from GFG Moderators there. Please choose 'ReadOnlyMode' if you needn't to 'Edit' the problem e. Please note that Custom Input s should be mentioned in the same order format as stated in the problem description. Cancel Send. Sign In Sign Up. Remember me Forgot Password. Why Create an Account? Please enter your email address or userHandle.

Queue using two Stacks. Login to solve this problem. Load Comments. Weekly Monthly Overall saiujwal vinup isiddhisingh rathiarpit29 devgoyal Leaderboard Overall.

Display over lanEditMode ReadOnlyMode. Close Run Code. Close See Output. Login to report an issue on this page. Note: Please use this button to report only Software related issues. For queries regarding questions and quizzes, use the comment area below respective pages. Describe Your Issue. Send Close. Ibrahim Nash.Stack is LIFO last in - first out data structure, in which elements are added and removed from the same end, called top.

In general stack is implemented using array or linked list, but in the current article we will review a different approach for implementing stack using queues. In contrast queue is FIFO first in - first out data structure, in which elements are added only from the one side - rear and removed from the other - front. In order to implement stack using queues, we need to maintain two queues q1 and q2. Also we will keep top stack element in a constant memory. The new element is always added to the rear of queue q1 and it is kept as top stack element.

Time complexity : O 1 O 1 O 1.

Queue is implemented as linked list and add operation has O 1 O 1 O 1 time complexity. Space complexity : O 1 O 1 O 1. We need to remove the element from the top of the stack. This is the last inserted element in q1.

Because queue is FIFO first in - first out data structure, the last inserted element could be removed only after all elements, except it, have been removed. For this reason we need to maintain additional queue q2which will serve as a temporary storage to enqueue the removed elements from q1.

The last inserted element in q2 is kept as top. Then the algorithm removes the last element in q1. We swap q1 with q2 to avoid copying all elements from q2 to q1. The algorithm inserts each new element to queue q2 and keep it as the top element. In case queue q1 is not empty there are elements in the stackwe remove all elements from q1 and add them to q2.

In this way the new inserted element top element in the stack will be always positioned at the front of q2.

Tarasov hockeyn mitt livTime complexity : O n O n O n. The operations add and remove in linked lists has O 1 O 1 O 1 complexity. The algorithm dequeues an element from queue q1 and keeps front element of q1 as top.

Fsx a340 300In both approaches empty and top operations have the same implementation. Queue q1 always contains all stack elements, so the algorithm checks q1 size to return if the stack is empty.

Freeze dryer priceThe top element is kept in constant memory and is modified each time when we push or pop an element. The top element has been calculated in advance and only returned in top operation. The mentioned above two approaches have one weakness, they use two queues. This could be optimized as we use only one queue, instead of two.

When we push an element into a queue, it will be stored at back of the queue due to queue's properties. But we need to implement a stack, where last inserted element should be in the front of the queue, not at the back. To achieve this we can invert the order of queue elements when pushing a new element.

The last inserted element is always stored at the front of q1 and we can pop it for constant time.

### Implement Stack using Queues

Queue q1 contains all stack elements, so the algorithm checks if q1 is empty. The top element is always positioned at the front of q1. Algorithm return it. For Approach 1, the pop method returns void when it should return the top. Code below puts the top in a temporary variable called theTop and then it gets removed after.The problem is opposite of this post.

We are given a Queue data structure that supports standard operations like enqueue and dequeue. We need to implement a Stack data structure using only instances of Queue and queue operations allowed on the instances. A stack can be implemented using two queues. Method 2 By making pop operation costly In push operation, the new element is always enqueued to q1. In pop operation, if q2 is empty then all the elements except the last, are moved to q2.

Finally the last element is dequeued from q1 and returned. References: Implement Stack using Two Queues. This article is compiled by Sumit Jain and reviewed by GeeksforGeeks team.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Writing code in comment? Please use ide. Stack s. Program to implement a stack using. Two inbuilt queues. To maintain current number.

Push x first in empty q2. Push all the remaining. This code is contributed by PranchalK. Enqueue x. Enqueue q1. Peek.

### Data Structure and Algorithms - Queue

Dequeue. WriteLine s. Enqueue res. Push 1. Top .

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