def merge(left, right):
    res = []
    i, j = 0, 0

    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            res.append(left[i])
            i+=1
        else:
            res.append(right[j])
            j+=1
    res += left[i:]
    res += right[j:]
    return res

def merge_sort(arr):
    if len(arr) <= 1:
        return arr
    mid = int(len(arr) / 2)
    left = merge_sort(arr[:mid])
    right = merge_sort(arr[mid:])
    return merge(left, right)

merge_sort([12,4,5,6,3,7,15,1])