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Add reverse() method for doubly linked list.

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src/data-structures/doubly-linked-list
- DoublyLinkedList.js
- __test__
  - DoublyLinkedList.test.js

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Original file line number	Diff line number	Diff line change
`@@ -230,4 +230,32 @@ export default class DoublyLinkedList {`
`230`	`230`	`toString(callback) {`
`231`	`231`	`return this.toArray().map(node => node.toString(callback)).toString();`
`232`	`232`	`}`
	`233`	`+`
	`234`	`+ /**`
	`235`	`+ * Reverse a linked list.`
	`236`	`+ * @returns {DoublyLinkedList}`
	`237`	`+ */`
	`238`	`+ reverse() {`
	`239`	`+ let currNode = this.head;`
	`240`	`+ let prevNode = null;`
	`241`	`+ let nextNode = null;`
	`242`	`+`
	`243`	`+ while (currNode) {`
	`244`	`+ // Store next node.`
	`245`	`+ nextNode = currNode.next;`
	`246`	`+`
	`247`	`+ // Change next node of the current node so it would link to previous node.`
	`248`	`+ currNode.next = prevNode;`
	`249`	`+`
	`250`	`+ // Move prevNode and currNode nodes one step forward.`
	`251`	`+ prevNode = currNode;`
	`252`	`+ currNode = nextNode;`
	`253`	`+ }`
	`254`	`+`
	`255`	`+ // Reset head and tail.`
	`256`	`+ this.tail = this.head;`
	`257`	`+ this.head = prevNode;`
	`258`	`+`
	`259`	`+ return this;`
	`260`	`+ }`
`233`	`261`	`}`

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Original file line number	Diff line number	Diff line change
`@@ -228,4 +228,30 @@ describe('DoublyLinkedList', () => {`
`228`	`228`	`expect(node.value.customValue).toBe('test2');`
`229`	`229`	`expect(linkedList.find({ value: 2, customValue: 'test5' })).toBeNull();`
`230`	`230`	`});`
	`231`	`+`
	`232`	`+ it('should reverse linked list', () => {`
	`233`	`+ const linkedList = new DoublyLinkedList();`
	`234`	`+`
	`235`	`+ // Add test values to linked list.`
	`236`	`+ linkedList`
	`237`	`+ .append(1)`
	`238`	`+ .append(2)`
	`239`	`+ .append(3);`
	`240`	`+`
	`241`	`+ expect(linkedList.toString()).toBe('1,2,3');`
	`242`	`+ expect(linkedList.head.value).toBe(1);`
	`243`	`+ expect(linkedList.tail.value).toBe(3);`
	`244`	`+`
	`245`	`+ // Reverse linked list.`
	`246`	`+ linkedList.reverse();`
	`247`	`+ expect(linkedList.toString()).toBe('3,2,1');`
	`248`	`+ expect(linkedList.head.value).toBe(3);`
	`249`	`+ expect(linkedList.tail.value).toBe(1);`
	`250`	`+`
	`251`	`+ // Reverse linked list back to initial state.`
	`252`	`+ linkedList.reverse();`
	`253`	`+ expect(linkedList.toString()).toBe('1,2,3');`
	`254`	`+ expect(linkedList.head.value).toBe(1);`
	`255`	`+ expect(linkedList.tail.value).toBe(3);`
	`256`	`+ });`
`231`	`257`	`});`

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