/**
 * 集合
 */
function Set() {
    var items = {};
	this.has = function(value){
        return items.hasOwnProperty(value);	
	};
	this.add = function(value){
    	if (!this.has(value)){
        	items[value] = value;
        	return true;
    	}
    	return false;
	};
	this.remove = function(value){
		if (this.has(value)){
			delete items[value];
			return true; 
        }
		return false;
	};
	this.clear = function(){
        items = {};
	};
	this.size = function(){
        return Object.keys(items).length;
	};
	this.values = function(){
    	return Object.keys(items);
	};
	// 并集
	this.union = function(otherSet){
        var unionSet = new Set();
        var values = this.values();
        for (var i=0; i<values.length; i++){
            unionSet.add(values[i]);
        }
        values = otherSet.values();
        for (var i=0; i<values.length; i++){
            unionSet.add(values[i]);
        }
        return unionSet;
    };
	// 交集
	this.intersection = function(otherSet){
        var intersectionSet = new Set();
        var values = this.values();
        for (var i=0; i<values.length; i++){
            if (otherSet.has(values[i])){
                intersectionSet.add(values[i]);
			} 
		}
        return intersectionSet;
    }
	// 差集
	this.difference = function(otherSet){
        var differenceSet = new Set();
        var values = this.values();
        for (var i=0; i<values.length; i++){
            if (!otherSet.has(values[i])){
				differenceSet.add(values[i]);
        	}
		}
    	return differenceSet;
	};
	// 子集
	this.subset = function(otherSet){
		if (this.size() > otherSet.size()){
        	return false;
    	} else {
        	var values = this.values();
        	for (var i=0; i<values.length; i++){
            	if (!otherSet.has(values[i])){
                	return false;
				} 
			}
        	return true;
    	}
	};
}