add notes on reversing

linked_lists/README.md

@@ -2,22 +2,69 @@
 
 <br>
 
-* unlike an array, a list does not provide constant time access to an index (as it needs to interact through all k elements), however addition and removal of elements are constant time.
+* each node in a singly-linked list contains a valye and a reference fiedl to link to the next node. the head node (first node) usually represents the whole list.
+
+* unlike an array, a linked list does not provide constant time access to an index (as it needs to interact through all k elements), however addition and removal of elements are constant time.
+
+* nodes can be added at the beginning, head needs to be update (`current -> head` and `head = current`).
+
 * to remove a node you set `prev.next` equal to `node.next`. if it's a double list, you also update `node.next` with `node.next.prev` to `node.prev` (and deallocate the memory).
 
 <br>
 
+
+--- 
+
 ### detecting cycles
 
 <br>
 
-* can be done with hash table or the two-pointer technique, where if there is no cycle, the faster pointer (going 2 steps) will stop at the end of the list, but if there is a cycle, the fast pointer will eventually meet the slow pointer (going 1 step). if there is no cycle, the faster pointer takes `N/2` to reach the end of the list (`N` being the length).
+* can be done with hash table or the two-pointer technique, where if there is no cycle, the faster pointer (going 2 steps) will stop at the end of the list, but if there is a cycle, the fast pointer will eventually meet the slow pointer (going 1 step).
+
+* if there is no cycle, the faster pointer takes `N/2` to reach the end of the list (`N` being the length).
 
 <br>
 
 ----
 
-### `LinkedListFIFO.py`
+### reversing the list
+
+<br>
+
+* keep track of the original head node and the new head node (for instance, with two pointers).
+
+<br>
+
+```python
+def reverse_list(head):
+
+  if head is None:
+      return head
+        
+  final_head = head
+        
+  while head.next:
+      
+      new_node = head.next
+      head.next = new_node.next
+      new_node.next = final_head
+      final_head = new_node
+        
+  return final_head
+```
+
+
+<br>
+
+<br>
+
+----
+
+### some exercises in this dir
+
+<br>
+
+#### `LinkedListFIFO.py`
 
 <br>