21草稿

function test(t1, t2) {

    const c1 = t1[0]
    const c2 = t2[0]

    if (c1 === null && c2 !== null) {
        arr.concat(t2)
        return arr
    }

    if (c2 === null && c1 !== null) {
        arr.concat(t1)
        return arr
    }

    if (c1 < c2) {
        arr.push(c1)
        t1.shift()
        return test(t1, t2)
    }

    if (c2 < c1) {
        arr.push(c2)
        t2.shift()
        return test(t1, t2)
    }
}