Iter: dr, cur, next; Recur: helper(ori, tail)

Author: GuanhuiGuan

Reverse Linked List.java

@@ -7,34 +7,32 @@
  * }
  */
 class Solution {
-    // recursion
     public ListNode reverseList(ListNode head) {
-        // use helper with two input nodes, one for attach at end, one for unprocessed part
-        return helper(head, null);
+        // return iterative(head);
+        return recursive(head);
     }
 
-    public ListNode helper(ListNode unPro, ListNode tail) {
-        // unPro: head of unprocessed; tail: what to attach at the tail
-        if(unPro == null) return tail;
-        ListNode nextUnPro = unPro.next;
-        unPro.next = tail;
-        ListNode newHead = helper(nextUnPro, unPro);
-        return newHead;
+    public ListNode iterative(ListNode head) {
+        ListNode dr = new ListNode(0);
+        ListNode cur = head;
+        
+        while(cur != null) {
+            ListNode next = cur.next;
+            cur.next = dr.next;
+            dr.next = cur;
+            cur = next;
+        }
+        return dr.next;
     }
 
+    public ListNode recursive(ListNode head) {
+        return recurHelper(head, null);
+    }
 
-    // iterative
-    public ListNode reverseList(ListNode head) {
-        if(head == null) return head;
-        // cur cannot be root
-        ListNode root = new ListNode(0), cur = head, next = head.next;
-        root.next = head;
-        while(next != null) {
-            cur.next = next.next;
-            next.next = root.next;
-            root.next = next;
-            next = cur.next;
-        }
-        return root.next;
+    public ListNode recurHelper(ListNode ori, ListNode tail) {
+        if(ori == null) return tail;
+        ListNode nextOri = ori.next;
+        ori.next = tail;
+        return recurHelper(nextOri, ori);
     }
 }