25-reverse-nodes-in-k-groups: Add Iterative Solution

java/25-Reverse-Nodes-in-k-Group.java

@@ -1,6 +1,20 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+
 //This recursive approach is super intuitive taken from leetcode discuss.
 
-class Solution {
+// Time Complexity: O(n)
+// Extra Space Complexity: O(n)
+
+class Solution1 {
 
     public ListNode reverseKGroup(ListNode head, int k) {
         ListNode cur = head;
@@ -22,3 +36,45 @@ public ListNode reverseKGroup(ListNode head, int k) {
         return head;
     }
 }
+
+// An iterative approach 
+
+// Time Complexity: O(n)
+// Extra Space Complexity: O(1)
+
+class Solution2 {
+    public ListNode reverseKGroup(ListNode head, int k) {
+        ListNode dummy = new ListNode(0, head);
+        ListNode curr = head;
+        ListNode prev = dummy;
+        ListNode temp = null;
+
+        int count = k;
+
+        while (curr != null) {
+            if (count > 1) {
+                temp = prev.next;
+                prev.next = curr.next;
+                curr.next = curr.next.next;
+                prev.next.next = temp;
+
+                count--;
+            } else {
+                prev = curr;
+                curr = curr.next;
+
+                ListNode end = curr;
+
+                for(int i = 0; i < k; ++i) {
+                    if (end == null) {
+                        return dummy.next;
+                    }
+                    end = end.next;
+                }
+                count = k;
+            }
+        }
+
+        return dummy.next;
+    }
+}