数组拍平

1. 递归实现

function flat1(arr) {
    let res = []

    arr.forEach(v => {
        if(Array.isArray(v)) {
            res = res.concat(flat1(v))
        } else {
            res.push(v)
        }
    })

    return res;
}

2. 递归+reduce

function flat2(arr) {
    return arr.reduce((prev, cur) => {
       return prev.concat(Array.isArray(cur) ? flat2(cur) : cur)
    }, [])
}