feat: add swift implementation to lcof2 problem: No.026

lcof2/剑指 Offer II 026. 重排链表/README.md

@@ -288,6 +288,74 @@ func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
 }
 ```
 
+#### Swift
+
+```swift
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     var val: Int
+ *     var next: ListNode?
+ *     init() { self.val = 0; self.next = nil; }
+ *     init(_ val: Int) { self.val = val; self.next = nil; }
+ *     init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
+ * }
+ */
+
+class Solution {
+    func reorderList(_ head: ListNode?) {
+        guard let head = head else { return }
+
+        let mid = middleNode(head)
+
+        let secondHalf = reverseList(mid.next)
+        mid.next = nil
+
+        mergeTwoLists(head, secondHalf)
+    }
+
+    private func middleNode(_ head: ListNode?) -> ListNode {
+        var slow = head
+        var fast = head
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+        }
+        return slow!
+    }
+
+    private func reverseList(_ head: ListNode?) -> ListNode? {
+        var prev: ListNode? = nil
+        var curr = head
+        while curr != nil {
+            let nextTemp = curr?.next
+            curr?.next = prev
+            prev = curr
+            curr = nextTemp
+        }
+        return prev
+    }
+
+    private func mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) {
+        var l1 = l1
+        var l2 = l2
+        while l1 != nil && l2 != nil {
+            let l1Next = l1?.next
+            let l2Next = l2?.next
+
+            l1?.next = l2
+            if l1Next == nil {
+                break
+            }
+            l2?.next = l1Next
+
+            l1 = l1Next
+            l2 = l2Next
+        }
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 026. 重排链表/Solution.swift

@@ -0,0 +1,63 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     var val: Int
+ *     var next: ListNode?
+ *     init() { self.val = 0; self.next = nil; }
+ *     init(_ val: Int) { self.val = val; self.next = nil; }
+ *     init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
+ * }
+ */
+
+class Solution {
+    func reorderList(_ head: ListNode?) {
+        guard let head = head else { return }
+
+        let mid = middleNode(head)
+
+        let secondHalf = reverseList(mid.next)
+        mid.next = nil
+
+        mergeTwoLists(head, secondHalf)
+    }
+
+    private func middleNode(_ head: ListNode?) -> ListNode {
+        var slow = head
+        var fast = head
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+        }
+        return slow!
+    }
+
+    private func reverseList(_ head: ListNode?) -> ListNode? {
+        var prev: ListNode? = nil
+        var curr = head
+        while curr != nil {
+            let nextTemp = curr?.next
+            curr?.next = prev
+            prev = curr
+            curr = nextTemp
+        }
+        return prev
+    }
+
+    private func mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) {
+        var l1 = l1
+        var l2 = l2
+        while l1 != nil && l2 != nil {
+            let l1Next = l1?.next
+            let l2Next = l2?.next
+
+            l1?.next = l2
+            if l1Next == nil {
+                break
+            }
+            l2?.next = l1Next
+
+            l1 = l1Next
+            l2 = l2Next
+        }
+    }
+}