错的那版，在已排好序的输入上给什么

下面 msort_bad 漏了合并的最后一句。先拿最顺手的那个测试用例——已经排好序的数组——试试：

def msort(a):
    if len(a) <= 1:
        return list(a)
    m = len(a) // 2
    L = msort(a[:m])
    Rr = msort(a[m:])
    out, i, j = [], 0, 0
    while i < len(L) and j < len(Rr):
        if L[i] <= Rr[j]:
            out.append(L[i]); i += 1
        else:
            out.append(Rr[j]); j += 1
    return out + L[i:] + Rr[j:]

def msort_bad(a):
    """漏了一句：合并到最后没把左半剩下的接上"""
    if len(a) <= 1:
        return list(a)
    m = len(a) // 2
    L = msort_bad(a[:m])
    Rr = msort_bad(a[m:])
    out, i, j = [], 0, 0
    while i < len(L) and j < len(Rr):
        if L[i] <= Rr[j]:
            out.append(L[i]); i += 1
        else:
            out.append(Rr[j]); j += 1
    return out + Rr[j:]

print("/".join(str(x) for x in msort_bad([0, 1, 2, 3, 4])))