Design a stack with operations on middle element

How to implement a stack which will support following operations in O(1) time complexity?
1) push() which adds an element to the top of stack.
2) pop() which removes an element from top of stack.
3) findMiddle() which will return middle element of the stack.
4) deleteMiddle() which will delete the middle element.
Push and pop are standard stack operations.

The important question is, whether to use a linked list or array for implementation of stack?

Please note that, we need to find and delete middle element. Deleting an element from middle is not O(1) for array. Also, we may need to move the middle pointer up when we push an element and move down when we pop(). In singly linked list, moving middle pointer in both directions is not possible.

The idea is to use Doubly Linked List (DLL). We can delete middle element in O(1) time by maintaining mid pointer. We can move mid pointer in both directions using previous and next pointers.

Following is C implementation of push(), pop() and findMiddle() operations. Implementation of deleteMiddle() is left as an exercise. If there are even elements in stack, findMiddle() returns the first middle element. For example, if stack contains {1, 2, 3, 4}, then findMiddle() would return 2.

/* Program to implement a stack that supports findMiddle() and deleteMiddle
   in O(1) time */
#include <stdio.h>
#include <stdlib.h>

/* A Doubly Linked List Node */
struct DLLNode
    struct DLLNode *prev;
    int data;
    struct DLLNode *next;

/* Representation of the stack data structure that supports findMiddle()
   in O(1) time.  The Stack is implemented using Doubly Linked List. It
   maintains pointer to head node, pointer to middle node and count of
   nodes */
struct myStack
    struct DLLNode *head;
    struct DLLNode *mid;
    int count;

/* Function to create the stack data structure */
struct myStack *createMyStack()
    struct myStack *ms =
               (struct myStack*) malloc(sizeof(struct myStack));
    ms->count = 0;
    return ms;

/* Function to push an element to the stack */
void push(struct myStack *ms, int new_data)
    /* allocate DLLNode and put in data */
    struct DLLNode* new_DLLNode =
               (struct DLLNode*) malloc(sizeof(struct DLLNode));
    new_DLLNode->data  = new_data;

    /* Since we are adding at the begining,
      prev is always NULL */
    new_DLLNode->prev = NULL;

    /* link the old list off the new DLLNode */
    new_DLLNode->next = ms->head;

    /* Increment count of items in stack */
    ms->count += 1;

    /* Change mid pointer in two cases
       1) Linked List is empty
       2) Number of nodes in linked list is odd */
    if (ms->count == 1)
         ms->mid = new_DLLNode;
        ms->head->prev = new_DLLNode;

        if (ms->count & 1) // Update mid if ms->count is odd
           ms->mid = ms->mid->prev;

    /* move head to point to the new DLLNode */
    ms->head  = new_DLLNode;

/* Function to pop an element from stack */
int pop(struct myStack *ms)
    /* Stack underflow */
    if (ms->count  ==  0)
        printf("Stack is empty\n");
        return -1;

    struct DLLNode *head = ms->head;
    int item = head->data;
    ms->head = head->next;

    // If linked list doesn't become empty, update prev
    // of new head as NULL
    if (ms->head != NULL)
        ms->head->prev = NULL;

    ms->count -= 1;

    // update the mid pointer when we have even number of
    // elements in the stack, i,e move down the mid pointer.
    if (!((ms->count) & 1 ))
        ms->mid = ms->mid->next;


    return item;

// Function for finding middle of the stack
int findMiddle(struct myStack *ms)
    if (ms->count  ==  0)
        printf("Stack is empty now\n");
        return -1;

    return ms->mid->data;

// Driver program to test functions of myStack
int main()
    /* Let us create a stack using push() operation*/
    struct myStack *ms = createMyStack();
    push(ms, 11);
    push(ms, 22);
    push(ms, 33);
    push(ms, 44);
    push(ms, 55);
    push(ms, 66);
    push(ms, 77);

    printf("Item popped is %d\n", pop(ms));
    printf("Item popped is %d\n", pop(ms));
    printf("Middle Element is %d\n", findMiddle(ms));
    return 0;


Item popped is 77
Item popped is 66
Middle Element is 33

This article is contributed by Chandra Prakash. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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