题目：将数字转换为每3位用,分隔的表示形式，例如1000 => 1,000
字符串方法：string.slice(start,end)
思路：将数字转换为字符串后，每3位插入一个,
let num = 1000000000000000000000000
let s = num + ''
let n = s.length
for(let i = 1; i <= Math.ceil(n / 3) - 1 ; i++){
s = s.slice(0, n - 3 * i) + ',' + s.slice(n - 3 * i)
}
console.log(s) //1,000,000,000,000,000,000,000,000
