去重

利用对象的属性不会重复这一特性，校验数组元素是否重复

distinct:function(a, b) {
    let arr = a.concat(b)
    let result = []
    let obj = {}
    for (let i of arr) {
        if (!obj[i]) {
            result.push(i)
            obj[i] = 1
        }
    }
    return result
},