chore: reverse linked list

Author: rupeshtiwari

README.md

@@ -959,6 +959,8 @@ Try `Binary Search`.
 
 #### Reverse linked list
 
+![](https://i.imgur.com/eNRiftX.png)
+
 ![](https://i.imgur.com/R9ecxwI.png)
 
 ![](https://i.imgur.com/iaJ66fu.png)

src/data-structure/2-linked-list/2-reverse-singly-linked-list.mjs

@@ -1,99 +1,63 @@
-import { creatLinkedListFromArray } from './1-implement-linked-list.mjs';
-import { display } from './1-implement-linked-list.mjs';
+class Node {
+  constructor(data) {
+    this.data = data;
+    this.next = null;
+  }
+}
 
-/**
- * 2->3->4->5->6->null
- * R  L
- *
- * 2->null
- *
- * 3->2->null
- * R
- * 4->5->6->null
- * L
- *
- * 4->3->2->null
- * R
- * 5->6->null
- * L
- *
- * 5->4->3->2->null
- * R
- * 6->null
- * L
- *
- * 6->5->4->3->2->null
- * R
- *
- */
+function insert(head, val) {
+  const node = new Node(val);
+  if (head == null) {
+    return node;
+  } else {
+    node.next = head;
+  }
 
-/**
- *  2->3->4->5->6
- * 
- *  R->2;
- *  L ->3; 
- *   
- *  R.next = null
- *  l points next 
- *      temp = L.next
-        L.next = R
-        R = L
-        L = temp
+  return node;
+}
 
- *  L.next = R
-    R = L            
- */
+function main() {
+  let head = null;
+  head = insert(head, 2);
+  head = insert(head, 4);
+  head = insert(head, 6);
+  head = insert(head, 8);
 
-/**
- * runtime complexity: O(n) because reverse the list in single loop
- * space/memory complexity: O(1) because it is iterative
- * @param head head
- * @returns reversed head
- */
-function reverse(head) {
-  let reverse = head;
-  let listToDo = head.next;
+  console.log(print(head));
+
+  return head;
+}
 
-  head.next = null;
+function print(head) {
+  if (!head) return '';
 
-  while (listToDo) {
-    let temp = listToDo.next;
-    listToDo.next = reverse;
-    reverse = listToDo;
-    listToDo = temp;
+  let current = head;
+  let s = '';
+
+  while (current.next) {
+    s += ' ' + current.data;
+    current = current.next;
   }
 
-  return reverse;
+  s += ' ' + current.data;
+
+  return s.trim();
 }
 
-const head = creatLinkedListFromArray([2, 3, 4, 5, 6]);
-const reversedHead = reverse(head);
-console.log(display(reversedHead));
+// O(n) time | O(1) space
+function reverse(head) {
+  let current = head;
+  let previous = null;
+  let next = null;
+
+  while (current) {
+    next = current.next;
+    current.next = previous;
+    previous = current;
+    current = next;
+  }
+
+  head = previous;
 
-/** TEST
- * 2-3-4-5-6
- * R L
- *
- * 2-null
- *
- * 3-2-null
- * R
- * 4-5-6-null
- * L
- *
- * 4-3-2-null
- * R
- * 5-6-null
- * L
- *
- * 5-4-3-2-null
- * R
- * 6-null
- * L
- *
- * 6-5-4-3-2-null
- * R
- *
- * null
- * L
- */
+  return head;
+}