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import LinkedList from '../linked-list/LinkedList';

export default class Stack {

constructor() {

// We're going to implement Stack based on LinkedList since these

// structures are quite similar. Compare push/pop operations of the Stack

// with prepend/deleteHead operations of LinkedList.

this.linkedList = new LinkedList();

}

/**

* @return {boolean}

*/

isEmpty() {

// The stack is empty if its linked list doesn't have a head.

return !this.linkedList.head;

}

/**

* @return {*}

*/

peek() {

if (this.isEmpty()) {

// If the linked list is empty then there is nothing to peek from.

return null;

}

// Just read the value from the start of linked list without deleting it.

return this.linkedList.head.value;

}

/**

* @param {*} value

*/

push(value) {

// Pushing means to lay the value on top of the stack. Therefore let's just add

// the new value at the start of the linked list.

this.linkedList.prepend(value);

}

/**

* @return {*}

*/

pop() {

// Let's try to delete the first node (the head) from the linked list.

// If there is no head (the linked list is empty) just return null.

const removedHead = this.linkedList.deleteHead();

return removedHead ? removedHead.value : null;

}

/**

* @return {*[]}

*/

toArray() {

return this.linkedList

.toArray()

.map((linkedListNode) => linkedListNode.value);

}

/**

* @param {function} [callback]

* @return {string}

*/

toString(callback) {

return this.linkedList.toString(callback);

}

}