var Stack = module.exports = function () {

this.head = null;

this.length = 0;

};

Stack.prototype.push = function (value) {

var node = {

value: value,

next: null

};

if (!this.head) {

this.head = node;

} else {

node.next = this.head;

this.head = node;

}

return this.length += 1;

};

Stack.prototype.pop = function () {

// If there is no head node, return `undefined`

if (!this.head) { return; }

var node = this.head;

// Update the head reference and remove the next node reference from the

// previous head.

this.head = node.next;

node.next = null;

this.length -= 1;

return node.value;

};