added problem of stack

Author: muskanmi

Stack/ImplementStack.java

@@ -0,0 +1,113 @@
+package Stack;
+
+public class ImplementStack {
+    
+    void push(int new_data)
+    {
+ 
+        /* allocate DLLNode and put in data */
+        DLLNode new_node = new DLLNode(new_data);
+        // if stack is empty
+        if (size == 0) {
+            head = new_node;
+            mid = new_node;
+            size++;
+            return;
+        }
+        head.next = new_node;
+        new_node.prev = head;
+ 
+        head = head.next;
+        if (size % 2 != 0) {
+            mid = mid.next;
+        }
+        size++;
+    }
+    int pop()
+    {
+        int data = -1;
+        /* Stack underflow */
+        if (size == 0) {
+            System.out.println("Stack is empty");
+            // return -1;
+        }
+ 
+        if (size != 0) {
+            data = head.data;
+            if (size == 1) {
+                head = null;
+                mid = null;
+            }
+            else {               
+                head = head.prev;
+                head.next = null;
+                if (size % 2 == 0) {
+                    mid = mid.prev;
+                }
+            }
+            size--;
+        }
+        return data;
+    }
+    int findMiddle()
+    {
+        if (size == 0) {
+            System.out.println("Stack is empty now");
+            return -1;
+        }
+        return mid.data;
+    }
+
+    void deleteMiddleElement()
+    {
+        // This function will not only delete the middle
+        // element
+        // but also update the mid in case of even and
+        // odd number of Elements
+        // when the size is even then findmiddle() will show the
+        // second middle element as mentioned in the problem
+        // statement
+        if (size != 0) {
+            if (size == 1) {
+                head = null;
+                mid = null;
+            }
+            else if (size == 2) {
+                head = head.prev;
+                mid = mid.prev;
+                head.next = null;
+            }
+            else {
+                mid.next.prev = mid.prev;
+                mid.prev.next = mid.next;
+                if (size % 2 == 0) {
+                    mid = mid.prev;
+                }
+                else {
+                    mid = mid.next;
+                }
+            }
+            size--;
+        }
+    }
+    public static void main(String[] args) {
+        myStack ms = new myStack();
+        ms.push(11);
+        ms.push(22);
+        ms.push(33);
+        ms.push(44);
+        ms.push(55);
+        ms.push(66);
+        ms.push(77);
+        ms.push(88);
+        ms.push(99);
+ 
+        System.out.println("Popped : " + ms.pop());
+        System.out.println("Popped : " + ms.pop());
+        System.out.println("Middle Element : "
+                           + ms.findMiddle());
+        ms.deleteMiddleElement();
+        System.out.println("New Middle Element : "
+                           + ms.findMiddle());
+    }
+}