集合（set类）

function Set() {
    let items = {};
    //has(value)方法
    //1
    this.has = function(value) {
        return value in items;
    };
    //2
    this.has = function(value) {
        return items.hasOwnProperty(value);
    };
    //add方法
    this.add = function(value) {
        if(!this.has(value)){
            items[value] = value;
            return true;
        }
        return true;
    };
    //remove方法
    this.remove = function(value) {
        if(this.has(value)) {
            delete items[value];
            return true;
        }
        return false;
    };
    //clear方法
    this.clear = function() {
        items = {}；
    };
    //size方法
    //1
    this.size = function() {
        return Object.keys(items).length;
    };
    //2
    this.size = function() {
        let count = 0;
        for(let key in items) {
            if(items.hasOwnProperty(key))
             ++count;
        }  
        return count;
    };
    //values方法
    //1
    this.values = function() {
        let values = [];
        for (let i=0, keys=Object.keys(items); i<keys.length; i++) {
            values.push(items[key]);
        }
        return values;
  };
  //2
  this.values = function() {
      let values = [];
      for(let key in items) {
          if(items.hasOwnProperty(key)) {
               values.push(items[key]);
          }
      }
      return values;
  }
}