实现flat

const arr = [1,2,3,[4,5,6,[7,8,9]] 平铺成一层结构[1,2,3,4,5,6,7,8,9]
1.递归
const newArr = []
function flatten(arr){
	arr.forEach(item => {
		if(Array.isArray(item)){
			flatten(item)
			return
		}
		newArr.push(item)
	})
	return newArr
}
2.reduce
function flatten(arr){
	return arr.reduce((result, item) => 
		result.concat(Array.isArray(item) ?  flatten(item) : item ),[])
}
3.join(toString)/split
function flatten(arr){
	return arr.join(',').split(',').map(item => {
			return parseInt(item)
	})
}