c: 0206-reverse-linked-list: use pointers instead of recursion

Recursion is hard to debug and can be difficult to follow.
Using struct pointers is faster and easier to understand.

c/0206-reverse-linked-list.c

@@ -1,24 +1,33 @@
-
 /**
+ * Given the head of a singly linked list, reverse the list, and return the
+ * reversed list.
+ *
+ * Constraints:
+ *
+ * The number of nodes in the list is the range [0, 5000].
+ * -5000 <= Node.val <= 5000
+ *
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     struct ListNode *next;
  * };
+ *
+ * Space: O(1)
+ * Time: O(n)
  */
 
-
 struct ListNode* reverseList(struct ListNode* head){
-
-    if (head == NULL) {
-        return NULL;
-    } else if (head -> next == NULL) {
-        return head;
+    struct ListNode *current = head;
+    struct ListNode *next = NULL;
+    struct ListNode *previous = NULL;
+
+    while (current) {
+        next = current->next;
+        current->next = previous;
+        previous = current;
+        current = next;
     }
-
-    struct ListNode* lastNode = reverseList(head -> next);
-    head -> next -> next = head;
-    head -> next = NULL;
-    return lastNode;
-
-}
+
+    return previous;
+}