[LinkedList] Add a solution to Reverse Nodes in k-Group

Author: DaiYue

LinkedList/ReverseNodesInKGroup.swift

@@ -0,0 +1,49 @@
+/**
+ * Question Link: https://leetcode.com/problems/reverse-nodes-in-k-group/
+ * Primary idea: Reverse one group during one loop. Construct a reversed group from back to forth.
+ * 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 ReverseNodesInKGroup {
+    func reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? {
+        let dummy = ListNode(0)
+        dummy.next = head
+        
+        var prev = dummy                             // prev = A
+        while prev.next != nil {                     // A->B->C->D->E => A->D->C->B->E
+            var groupTail : ListNode? = prev
+            for _ in 1...k {
+                groupTail = groupTail?.next
+            }
+            guard groupTail != nil else {            // groupTail = D
+                break
+            }
+            
+            let nextGroupHead = groupTail!.next      // nextGroupHead = E
+            var last = nextGroupHead                 // last = E
+            var current : ListNode? = prev.next!     // current = B
+            while current != nil && current !== nextGroupHead {
+                let next = current!.next             // next = C
+                current!.next = last                 // B -> E
+                last = current                       // last = B
+                current = next                       // current = C
+            }
+            
+            groupTail = prev.next
+            prev.next = last
+            prev = groupTail!
+        }
+        
+        return dummy.next
+    }
+}