[LinkedList] add Solution to Reorder List

LinkedList/ReorderList.swift

@@ -0,0 +1,67 @@
+/**
+ * Question Link: https://leetcode.com/problems/reorder-list/
+ * Primary idea: Use Runner Tech to split the list, reverse the second half, and merge them
+ * 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 ReorderList {
+    func reorderList(head: ListNode?) {
+        if head == nil || head!.next == nil {
+            return
+        }
+
+        // split the list
+        var prev = head
+        var post = head!.next
+
+        while post != nil && post!.next != nil {
+            prev = prev!.next
+            post = post!.next!.next
+        }
+
+        post = prev!.next
+        prev!.next = nil
+        prev = head
+
+        // reverse the second list
+        post = _reverse(post)
+
+        // merge lists
+        while prev != nil && post != nil{
+            let preNext = prev!.next
+            let posNext = post!.next
+
+            prev!.next = post
+            post!.next = preNext
+
+            prev = preNext
+            post = posNext
+        }
+    }
+
+    private func _reverse(head: ListNode?) -> ListNode?{
+        var prev = head
+        var temp: ListNode?
+
+        while prev != nil {
+            let post = prev!.next
+
+            prev!.next = temp
+
+            temp = prev
+            prev = post
+        }
+
+        return temp
+    }
+}