[LinkedList] Add a solution to Odd Even Linked List

Author: Yi Gu

LinkedList/OddEvenLinkedList.swift

@@ -0,0 +1,47 @@
+/**
+ * Question Link: https://leetcode.com/problems/odd-even-linked-list/
+ * Primary idea: Prev-post two pointers; change the prev and move both at a time
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.next = nil
+ *     }
+ * }
+ */
+
+class OddEvenLinkedList {
+    func oddEvenList(_ head: ListNode?) -> ListNode? {
+        guard head != nil && head!.next != nil else {
+            return head
+        }
+        
+        let evenHead = head!.next
+        var p = head
+        var q = evenHead
+        var isEndEven = true
+        
+        while q!.next != nil {
+            let node = q!.next
+            
+            p!.next = node
+            
+            p = q
+            q = node
+            isEndEven = !isEndEven
+        }
+        
+        if isEndEven {
+            p!.next = evenHead
+        } else {
+            p!.next = nil
+            q!.next = evenHead
+        }
+        
+        return head
+    }
+}