[Algorithm] Linked List Data Structure in JavaScript
A linked list is a collection of items where each item points to the next one in the list. Because of this structure, linked lists are very slow when searching for an item at a particular index. An array, by comparison, has quick get
s when searching for an index, but a linked list must start at the beginning, often called the "head", and loop through each item's next
property until we arrive at the item. This makes get
s in a linked list an operation that takes O(n)
time.
While get
s might be slow in a linked list, it's other operations, like push
and delete
come with some great benefits we will see in the lesson.
/**
* Linked list
*
* API:
* push
* pop
* get
* delete
* isEmpty
*/ function createNode(value) {
return {
value,
next: null
}
} function createLinkedList() {
return {
head: null,
tail: null,
length: 0,
push(value) {
/**Key takeaway:
* Assign new node to current tail's next value
* Then
* Reassign the tail to new node
*/
// Create Node
const node = createNode(value); // If this is the first one
if (this.head === null) {
this.head = node
this.tail = node
this.length++;
return node;
} // if there already has nodes
this.tail.next = node;
this.tail = node;
this.length++;
return node;
},
pop() {
const node = this.tail;
// if this is no node
if (!this.head) {
return null;
} // if there is one node
if (this.head === this.tail) {
this.head = null;
this.tail = null;
return node;
} let current = this.head;
let penultimate = null; while (current) {
const {next} = current;
if (next && next == this.tail) {
penultimate = current;
break;
}
current = current.next;
}
penultimate.next = null;
this.tail = penultimate;
this.length--;
return node;
},
get(index = 0) {
// no node in the list, return null
if (!this.head) {
return null;
} // if the index < 0 or > length - 1, out of range
if (index < 0 || index > this.length - 1) {
return null;
} // if index = 0, then return the first
if (index === 0) {
return this.head;
} let current = this.head;
let i = 0;
while (i < index) {
i++;
current = current.next;
} return current;
},
delete(index = 0) {
/**
* Key takewawy:
* If we delete tail, we need to reassign the tail
*/
// no node in the list, return null
if (!this.head) {
return null;
} // if the index < 0 or > length - 1, out of range
if (index < 0 || index > this.length - 1) {
return null;
} // if index = 0, then return the first
if (index === 0) {
const node = this.head;
this.head = node.next;
this.length--;
return node;
} let i = 0;
let current = this.head;
let previous = null; while (i < index) {
i++;
previous = current;
current = current.next;
} const deleted = current;
previous.next = deleted.next; // If we delete the tail, we need to reassign tail
if (previous.next === null) {
this.tail = previous;
} this.length--;
return deleted;
},
isEmpty() {
return this.length === 0;
},
print() {
/**Key takeway:
* remember to assign next node to current
* Move the while loop
* */
let nodes = []; if (!this.head) {
return 'Empty list';
} let current = this.head;
while (current) {
nodes.push(current.value);
current = current.next;
} return nodes.join(' => ');
}
};
} module.exports = {createLinkedList}
test:
const {createLinkedList} = require('../src/linked-list'); describe('linked list', () => {
test('push: should add node into array', () => {
const l = createLinkedList();
// linked list should be empty
expect(l.isEmpty()).toBe(true);
// push a new node
l.push('a');
expect(l.isEmpty()).toBe(false);
expect(l.length).toEqual(1);
expect(l.print()).toEqual('a');
// push a second node
l.push('b');
expect(l.length).toEqual(2);
expect(l.print()).toEqual('a => b');
}); test('pop: should remove the last node from the list', () => {
const l = createLinkedList();
l.push('a');
l.push('b');
l.push('c');
expect(l.length).toEqual(3);
const p = l.pop();
expect(p.value).toEqual('c');
expect(l.length).toEqual(2);
}); test('get: should return the node for the given index', () => {
const l = createLinkedList();
// empty list, return null
expect(l.get(0)).toBeNull();
l.push('a');
l.push('b');
l.push('c');
expect(l.length).toEqual(3);
// out of index, retur null
expect(l.get(-1)).toBeNull();
expect(l.get(4)).toBeNull(); // return the head
expect(l.get(0).value).toEqual('a'); // index in range not head
expect(l.get(2).value).toEqual('c');
}); test('delete: should delete the node from the given index', () => {
const l = createLinkedList();
// empty list, return null
expect(l.delete(0)).toBeNull();
l.push('a');
l.push('b');
l.push('c');
expect(l.length).toEqual(3);
// out of index, retur null
expect(l.delete(-1)).toBeNull();
expect(l.delete(4)).toBeNull();
// return the head
expect(l.delete(0).value).toEqual('a');
expect(l.length).toEqual(2);
// delete the tail, reassign the tail
expect(l.delete(1).value).toEqual('c');
expect(l.tail.value).toEqual('b');
});
});
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