added a Linked List problem and it's solution

Added Even after Odd node problem

data_structures/Linked_list/C++/evenAfterOdd.cpp

@@ -0,0 +1,96 @@
+// Problem Statement: Given a Linked List of integers, write a
+// function to modify the linked list such that all odd numbers
+// appear before all the even numbers in the modified linked list.
+//  Also, keep the order of even and odd numbers the same.
+
+// Example:
+
+// Input: 1-->2-->3-->4-->5-->6-->NULL
+// Output: 1-->3-->5-->2-->4-->6-->NULL
+
+// Solution:
+
+#include <iostream>
+using namespace std;
+
+class node
+{
+public:
+    int data;
+    node *next;
+
+    node(int val)
+    {
+        data = val;
+        next = NULL;
+    }
+};
+
+node *evenAfterOdd(node *&head)
+{
+    node *temp = head;
+
+    node *dummy1 = new node(-1);
+    node *ptr1 = dummy1;
+
+    node *dummy2 = new node(-1);
+    node *ptr2 = dummy2;
+
+    int count = 1;
+    while (temp != NULL)
+    {
+        // odd in dummy1:
+        if (count % 2 == 1)
+        {
+            ptr1->next = temp;
+            ptr1 = ptr1->next;
+        }
+
+        // even in dummy2:
+        else
+        {
+            ptr2->next = temp;
+            ptr2 = ptr2->next;
+        }
+
+        count++;
+        temp = temp->next;
+    }
+
+    // combining dummy 1 and 2:
+    ptr1->next = dummy2->next;
+
+    return dummy1->next;
+}
+
+void printLL(node *head)
+{
+    while (head != NULL)
+    {
+        cout << head->data << "-->";
+        head = head->next;
+    }
+    cout << "NULL" << endl;
+}
+
+int main()
+{
+    node a(1);
+
+    node *head = &a;
+    node *temp = head;
+
+    for (int i = 1; i < 6; i++)
+    {
+        temp->next = new node(i + 1);
+        temp = temp->next;
+    }
+
+    printLL(head);
+
+    node *newHead = evenAfterOdd(head);
+
+    printLL(newHead);
+
+    return 0;
+}