Move linked list traversals into separate section.

Author: trekhleb

README.md

@@ -109,6 +109,9 @@ a set of rules that precisely define a sequence of operations.
   * `B` [Shellsort](src/algorithms/sorting/shell-sort)
   * `B` [Counting Sort](src/algorithms/sorting/counting-sort)
   * `B` [Radix Sort](src/algorithms/sorting/radix-sort)
+* **Linked Lists**
+  * `B` [Straight Traversal](src/algorithms/linked-list/traversal)
+  * `B` [Reverse Traversal](src/algorithms/linked-list/reverse-traversal)
 * **Trees**
   * `B` [Depth-First Search](src/algorithms/tree/depth-first-search) (DFS)
   * `B` [Breadth-First Search](src/algorithms/tree/breadth-first-search) (BFS)

src/algorithms/linked-list/reverse-traversal/README.md

@@ -0,0 +1,19 @@
+# Reversed Linked List Traversal
+
+The task is to traverse the given linked list in reversed order.
+
+For example for the following linked list: 
+
+![](https://upload.wikimedia.org/wikipedia/commons/6/6d/Singly-linked-list.svg)
+
+The order of traversal should be:
+
+```text
+37 → 99 → 12
+```
+
+The time complexity is `O(n)` because we visit every node only once.
+
+## Reference
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Linked_list)

src/algorithms/linked-list/reverse-traversal/__test__/reverseTraversal.test.js

@@ -0,0 +1,36 @@
+import LinkedList from '../../../../data-structures/linked-list/LinkedList';
+import reverseTraversal from '../reverseTraversal';
+
+describe('reverseTraversal', () => {
+  it('should traverse linked list in reverse order', () => {
+    const linkedList = new LinkedList();
+
+    linkedList
+      .append(1)
+      .append(2)
+      .append(3);
+
+    const traversedNodeValues = [];
+    const traversalCallback = (nodeValue) => {
+      traversedNodeValues.push(nodeValue);
+    };
+
+    reverseTraversal(linkedList, traversalCallback);
+
+    expect(traversedNodeValues).toEqual([3, 2, 1]);
+  });
+});
+
+
+// it('should reverse traversal the linked list with callback', () => {
+//   const linkedList = new LinkedList();
+//
+//   linkedList
+//     .append(1)
+//     .append(2)
+//     .append(3);
+//
+//   expect(linkedList.toString()).toBe('1,2,3');
+//   expect(linkedList.reverseTraversal(linkedList.head, value => value * 2)).toEqual([6, 4, 2]);
+//   expect(() => linkedList.reverseTraversal(linkedList.head)).toThrow();
+// });

src/algorithms/linked-list/reverse-traversal/reverseTraversal.js

@@ -0,0 +1,24 @@
+/**
+ * Traversal callback function.
+ * @callback traversalCallback
+ * @param {*} nodeValue
+ */
+
+/**
+ * @param {LinkedListNode} node
+ * @param {traversalCallback} callback
+ */
+function reverseTraversalRecursive(node, callback) {
+  if (node) {
+    reverseTraversalRecursive(node.next, callback);
+    callback(node.value);
+  }
+}
+
+/**
+ * @param {LinkedList} linkedList
+ * @param {traversalCallback} callback
+ */
+export default function reverseTraversal(linkedList, callback) {
+  reverseTraversalRecursive(linkedList.head, callback);
+}

src/algorithms/linked-list/traversal/README.md

@@ -0,0 +1,19 @@
+# Linked List Traversal
+
+The task is to traverse the given linked list in straight order.
+
+For example for the following linked list: 
+
+![](https://upload.wikimedia.org/wikipedia/commons/6/6d/Singly-linked-list.svg)
+
+The order of traversal should be:
+
+```text
+12 → 99 → 37
+```
+
+The time complexity is `O(n)` because we visit every node only once.
+
+## Reference
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Linked_list)

src/algorithms/linked-list/traversal/__test__/traversal.test.js

@@ -0,0 +1,22 @@
+import LinkedList from '../../../../data-structures/linked-list/LinkedList';
+import traversal from '../traversal';
+
+describe('traversal', () => {
+  it('should traverse linked list', () => {
+    const linkedList = new LinkedList();
+
+    linkedList
+      .append(1)
+      .append(2)
+      .append(3);
+
+    const traversedNodeValues = [];
+    const traversalCallback = (nodeValue) => {
+      traversedNodeValues.push(nodeValue);
+    };
+
+    traversal(linkedList, traversalCallback);
+
+    expect(traversedNodeValues).toEqual([1, 2, 3]);
+  });
+});

src/algorithms/linked-list/traversal/traversal.js

@@ -0,0 +1,18 @@
+/**
+ * Traversal callback function.
+ * @callback traversalCallback
+ * @param {*} nodeValue
+ */
+
+/**
+ * @param {LinkedList} linkedList
+ * @param {traversalCallback} callback
+ */
+export default function traversal(linkedList, callback) {
+  let currentNode = linkedList.head;
+
+  while (currentNode) {
+    callback(currentNode.value);
+    currentNode = currentNode.next;
+  }
+}

src/data-structures/linked-list/LinkedList.js

@@ -208,40 +208,6 @@ export default class LinkedList {
     return this.toArray().map(node => node.toString(callback)).toString();
   }
 
-  /**
-   * Traverse through all nodes of the list from head to tail
-   * @param {*} callback
-   * @return {LinkedListNode[]}
-   */
-  traverse(callback = undefined) {
-    if (typeof callback !== 'function') {
-      throw new TypeError(`traverse method requires a callback function as an argument.\nArgument given: ${typeof callback}`);
-    }
-
-    let currentNode = this.head;
-    const traversedNodes = [];
-
-    while (currentNode) {
-      traversedNodes.push(callback(currentNode.value));
-      currentNode = currentNode.next;
-    }
-
-    return traversedNodes;
-  }
-
-  /**
-   * The items in the list have been traversed in reverse order
-   */
-  reverseTraversal(node, callback = undefined) {
-    if (typeof callback !== 'function') {
-      throw new TypeError(`reverseTraverse method requires a callback function as an argument.\nArgument given: ${typeof callback}`);
-    }
-
-    if (!node) return [];
-
-    return this.reverseTraversal(node.next, callback).concat(callback(node.value));
-  }
-
   /**
    * Reverse a linked list.
    * @returns {LinkedList}

src/data-structures/linked-list/__test__/LinkedList.test.js

@@ -218,31 +218,6 @@ describe('LinkedList', () => {
     expect(linkedList.find({ value: 2, customValue: 'test5' })).toBeNull();
   });
 
-  it('should traverse through all nodes of the list from head to tail with callback', () => {
-    const linkedList = new LinkedList();
-
-    linkedList
-      .append(1)
-      .append(2)
-      .append(3);
-
-    expect(linkedList.traverse(value => value * 2)).toEqual([2, 4, 6]);
-    expect(() => linkedList.traverse()).toThrow();
-  });
-
-  it('should reverse traversal the linked list with callback', () => {
-    const linkedList = new LinkedList();
-
-    linkedList
-      .append(1)
-      .append(2)
-      .append(3);
-
-    expect(linkedList.toString()).toBe('1,2,3');
-    expect(linkedList.reverseTraversal(linkedList.head, value => value * 2)).toEqual([6, 4, 2]);
-    expect(() => linkedList.reverseTraversal(linkedList.head)).toThrow();
-  });
-
   it('should reverse linked list', () => {
     const linkedList = new LinkedList();
 
@@ -258,9 +233,14 @@ describe('LinkedList', () => {
 
     // Reverse linked list.
     linkedList.reverse();
-
     expect(linkedList.toString()).toBe('3,2,1');
     expect(linkedList.head.value).toBe(3);
     expect(linkedList.tail.value).toBe(1);
+
+    // Reverse linked list back to initial state.
+    linkedList.reverse();
+    expect(linkedList.toString()).toBe('1,2,3');
+    expect(linkedList.head.value).toBe(1);
+    expect(linkedList.tail.value).toBe(3);
   });
 });