最小的k个数

#返回以随机选取的数partition之后的索引
#arr为数组，start、end分别为数组第一个元素和最后一个元素的索引，povitIndex为数组中任意选中的数的索引
def partition2(arr, start, end):
    pivotIndex = random.randint(start, end)
    arr[pivotIndex], arr[end] = arr[end], arr[pivotIndex]
    storeIndex = start
    #这个循环比一般的写法简洁高效，呵呵维基百科上看到的
    for i in range(start, end):
        if arr[i] <= arr[end]:
            arr[i], arr[storeIndex] = arr[storeIndex], arr[i]
            storeIndex += 1
    arr[storeIndex], arr[end] = arr[end], arr[storeIndex]
    return storeIndex

def MinK(arr, k):
    if arr is None or len(arr) < k or k <= 0:
        return None
    start = 0
    end = len(arr) -1
    index = partition2(arr, start, end)
    while index != k - 1:
        if index > k - 1:
            end = index - 1
            index = partition2(arr, start, end)
        else:
            start = index + 1
            index = partition2(arr, start, end)
    return arr[:k]

l = [7, 3, 1, 5, 6, 2]
l1 = [7, 3 ,1, 1, 1, 2]
print(MinK(l1, 3))