stack mid element

Author: sachuverma

DSA Crack Sheet/README.md

@@ -145,6 +145,7 @@
 - [Implement stack using array](https://practice.geeksforgeeks.org/problems/implement-stack-using-array/1# "view question") - [Cpp Solution](./solutions/Implement%20stack%20using%20array.cpp)
 - [Implement Queue using array](https://practice.geeksforgeeks.org/problems/implement-queue-using-array/1# "view question") - [Cpp Solution](./solutions/Implement%20Queue%20using%20array.cpp)
 - [Implement two stacks in an array](https://practice.geeksforgeeks.org/problems/implement-two-stacks-in-an-array/1# "view question") - [Cpp Solution](./solutions/Implement%20two%20stacks%20in%20an%20array.cpp)
+- [Design a stack with operations on middle element](https://www.geeksforgeeks.org/design-a-stack-with-find-middle-operation/ "view question") - [Cpp Solution](./solutions/Mid%20Element%20Stack.cpp)
 - []( "view question") - [Cpp Solution](./solutions/.cpp)
 
 ### Heap

DSA Crack Sheet/solutions/Mid Element Stack.cpp

@@ -0,0 +1,155 @@
+/*
+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. 
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+/* A Doubly Linked List Node */
+class DLLNode
+{
+public:
+  DLLNode *prev;
+  int data;
+  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 */
+class myStack
+{
+public:
+  DLLNode *head;
+  DLLNode *mid;
+  int count;
+};
+
+/* Function to create the stack data structure */
+myStack *createMyStack()
+{
+  myStack *ms = new myStack();
+  ms->count = 0;
+  return ms;
+};
+
+/* Function to push an element to the stack */
+void push(myStack *ms, int new_data)
+{
+  /* allocate DLLNode and put in data */
+  DLLNode *new_DLLNode = new DLLNode();
+  new_DLLNode->data = new_data;
+
+  /* Since we are adding at the beginning,
+	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;
+  }
+  else
+  {
+    ms->head->prev = new_DLLNode;
+
+    if (!(ms->count & 1)) // Update mid if ms->count is even
+      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(myStack *ms)
+{
+  /* Stack underflow */
+  if (ms->count == 0)
+  {
+    cout << "Stack is empty\n";
+    return -1;
+  }
+
+  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 odd number of
+  // elements in the stack, i,e
+  // move down the mid pointer.
+  if ((ms->count) & 1)
+    ms->mid = ms->mid->next;
+
+  free(head);
+
+  return item;
+}
+
+// Function for finding middle of the stack
+int findMiddle(myStack *ms)
+{
+  if (ms->count == 0)
+  {
+    cout << "Stack is empty now\n";
+    return -1;
+  }
+
+  return ms->mid->data;
+}
+// Function for deleting middle of the stack
+int deletemiddle(myStack *ms)
+{
+  int temp = ms->mid->data;
+  ms->mid->prev->next = ms->mid->next;
+  ms->mid->next->prev = ms->mid->prev->next;
+
+  delete ms->mid;
+  return temp;
+}
+
+int main()
+{
+  /* Let us create a stack using push() operation*/
+  myStack *ms = createMyStack();
+  push(ms, 11);
+  push(ms, 22);
+  push(ms, 33);
+  push(ms, 44);
+  push(ms, 55);
+  push(ms, 66);
+  push(ms, 77);
+
+  cout << "Item popped is " << pop(ms) << endl;
+  cout << "Item popped is " << pop(ms) << endl;
+  cout << "Middle Element is " << findMiddle(ms) << endl;
+  cout << "deleted mid element " << deletemiddle(ms) << endl;
+  return 0;
+}