封裝成函數(shù)：

//自頂向下歸并排序
func MergeSortUpToDown(s []int) {
    aux := make([]int, len(s)) //輔助切片
    mergeSortUpToDown(s, aux, 0, len(s)-1)
}
 
//自底向上歸并排序
func MergeSortDownToUp(s []int) {
    aux := make([]int, len(s)) //輔助切片
    n := len(s)
    for sz := 1; sz < n; sz *= 2 {
        for lo := 0; lo < n-sz; lo += 2 * sz {
            merge(s, aux, lo, lo+sz-1, min(lo+2*sz-1, n-1))
        }
    }
}
 
func mergeSortUpToDown(s, aux []int, lo, hi int) {
    if lo >= hi {
        return
    }
    mid := (lo + hi) >> 1
    mergeSortUpToDown(s, aux, lo, mid)
    mergeSortUpToDown(s, aux, mid+1, hi)
    merge(s, aux, lo, mid, hi)
}
 
//歸并操作
func merge(s, aux []int, lo, mid, hi int) {
    for k := lo; k <= hi; k++ {
        aux[k] = s[k]
    }
    i, j := lo, mid+1
    for k := lo; k <= hi; k++ {
        if i > mid {
            s[k] = aux[j]
            j++
        } else if j > hi {
            s[k] = aux[i]
            i++
        } else if aux[j] < aux[i] {
            s[k] = aux[j]
            j++
        } else {
            s[k] = aux[i]
            i++
        }
    }
}
 
func min(i, j int) int {
    if j < i {
        return j
    }
    return i
}

測試：

s := []int{9,0,6,5,8,2,1,7,4,3}
fmt.Println(s)
MergeSortUpToDown(s)
//MergeSortDownToUp(s)
fmt.Println(s)

輸出：
[9 0 6 5 8 2 1 7 4 3]
[0 1 2 3 4 5 6 7 8 9]