reorder list solution

Author: yhkee0404

reorder-list/yhkee0404.swift

@@ -0,0 +1,37 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init() { self.val = 0; self.next = nil; }
+ *     public init(_ val: Int) { self.val = val; self.next = nil; }
+ *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
+ * }
+ */
+class Solution {
+    func reorderList(_ head: ListNode?) {
+        let temp = ListNode(0, head)
+        solve(temp, temp, false)
+    }
+    func solve(_ tortoise: ListNode, _ hare: ListNode?, _ odd: Bool) -> ListNode? {
+        guard let safeHare = hare else {
+            let tail = ListNode(0, tortoise.next)
+            tortoise.next = nil
+            return tail
+        }
+        let tail = solve(odd ? tortoise : tortoise.next!, safeHare.next, !odd)
+        if !odd {
+            return tail
+        }
+        guard let safeTail = tail else {
+            return nil
+        }
+        if safeTail.val == 0 {
+            return safeTail.next
+        }
+        let next = safeTail.next
+        safeTail.next = tortoise.next
+        tortoise.next = safeTail
+        return next
+    }
+}