[refactor] change to md format

Author: keineahnung2345

206. Reverse Linked List.cpp renamed to 206. Reverse Linked List.md

@@ -1,4 +1,5 @@
-/**
+## Problem description
+
 Reverse a singly linked list.
 
 Example:
@@ -8,8 +9,10 @@ Output: 5->4->3->2->1->NULL
 Follow up:
 
 A linked list can be reversed either iteratively or recursively. Could you implement both?
-**/
 
+## Iterative
+
+```cpp
 //iterative
 /**
  * Definition for singly-linked list.
@@ -37,12 +40,12 @@ class Solution {
         return prev;
     }
 };
+```
 
+## Recursive
+The recursive version is slightly trickier and the key is to work backwards. Assume that the rest of the list had already been reversed, now how do I reverse the front part? 
 
-//recursive
-/**
-Approach #2 (Recursive) [Accepted]
-The recursive version is slightly trickier and the key is to work backwards. Assume that the rest of the list had already been reversed, now how do I reverse the front part? Let's assume the list is: n1 → … → nk-1 → nk → nk+1 → … → nm → Ø
+Let's assume the list is: n1 → … → nk-1 → nk → nk+1 → … → nm → Ø
 
 Assume from node nk+1 to nm had been reversed and you are at node nk.
 
@@ -57,9 +60,7 @@ nk.next.next = nk;
 Be very careful that n1's next must point to Ø. 
 If you forget about this, your linked list has a cycle in it. 
 This bug could be caught if you test your code with a linked list of size 2.
-**/
 
-/**
 Complexity analysis
 
 Time complexity : O(n). 
@@ -68,13 +69,13 @@ Assume that nn is the list's length, the time complexity is O(n).
 Space complexity : O(n).
 The extra space comes from implicit stack space due to recursion. 
 The recursion could go up to nn levels deep.
-**/
 
-/**
+
 Runtime: 8 ms, faster than 100.00% of C++ online submissions for Reverse Linked List.
+
 Memory Usage: 9.3 MB, less than 10.03% of C++ online submissions for Reverse Linked List.
-**/
-/**
+
+```cpp
 class Solution {
 public:
     ListNode* reverseList(ListNode* head) {
@@ -87,4 +88,4 @@ class Solution {
         return p;
     }
 };
-**/
+```