function Stack() {

   this.dataStore = [];

   this.top = 0;

   this.push = push;

   this.pop = pop;

   this.peek = peek;

   this.length = length;

   this.clear =clear;

}

function push(elem) {

   this.dataStore[this.top++] = elem;

}

function pop() {

	var data = this.dataStore[--this.top];

	this.dataStore.length = this.top;

   return data;

}

function peek() {

   return this.dataStore[--this.top];

}

function length() {

   return this.top;

}

function clear() {

   this.top = 0;

}