vault backup: 2022-06-15 23:14:20

CS notes/pages/Leetcode Reverse-Linked-List.md

@@ -8,7 +8,7 @@
 ##### Data structures:
 #DS #linked_list 
 ##### Difficulty:
-#leetcode  #coding_problem #difficulty-easy 
+#leetcode  #coding_problem #difficulty-medium
 ##### Lists:
 #CS_list_need_understanding #CS_list_need_practicing 
 ##### Related topics:
@@ -62,6 +62,8 @@ The in place insert is easier to understand, and simple to implement, using a ve
 ### Solution
 I've referred to this guy: https://leetcode.com/problems/reverse-linked-list/discuss/58130/C%2B%2B-Iterative-and-Recursive
 
+This code is hard to understand.
+==TODO==: Make my own version
 ```cpp
 class Solution {
 public:
@@ -70,14 +72,35 @@ public:
     // pre is before head, and insert any element after pre.
     pre->next = head;
     while (cur && cur->next) {
-      // Move cur->next after pre.
+      // temp points to head
       ListNode *temp = pre->next;
+      // Move cur->next after pre.
       pre->next = cur->next;
+      // Fix pointers, because cur->next is unchanged when changing position.
       cur->next = cur->next->next;
-
       pre->next->next = temp;
     }
     return pre->next;
   }
 };
 ```
+
+Recursion:
+```cpp
+class Solution {
+public:
+  ListNode *reverseList(ListNode *head) {
+    // Base case: reached the end of list
+    if (!head || !(head->next)) {
+      return head;
+    }
+
+    // node is the end of linked list, which stays the same and untouched
+    ListNode *node = reverseList(head->next);
+    // change head->next
+    head->next->next = head;
+    head->next = NULL;
+    return node;
+  }
+};
+```