Add OddEvenListReordering class and method

Implement odd-even reordering for a singly linked list.

Author: harshithasudhakar

Linked List/Singly Linked List/OddEvenListReordering.java

@@ -0,0 +1,39 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class OddEvenListReordering{
+
+    public ListNode oddEvenList(ListNode head) {
+
+        if(head == null || head.next == null){
+            return head;
+        }
+        
+        ListNode odd = head;
+        ListNode even = head.next;
+        ListNode evenHead = even;
+        
+        while(even!= null && even.next!=null){ //only checking even cause even will hit the end before odd cause it's a step ahead always
+            if(odd.next != null){
+                odd.next = odd.next.next;
+                odd = odd.next;
+            }
+        
+            if(even.next != null){
+                even.next = even.next.next;
+                even = even.next;
+            }
+        }
+
+        odd.next = evenHead;
+
+        return head;
+    }
+}