二分查找的循环和递归Python实现

二分查找的前提是数组或列表有序，下面以升序列表为例，key为要查找的关键字，查找成功返回下标，不成功返回None，给出循环和递归实现。

循环实现：

def HalfSearch(OrderedList, key, left, right):
    while left <= right:
        mid = (left + right) // 2
        if key == OrderedList[mid]:
            return mid
        elif key > OrderedList[mid]:
            left = mid + 1
        else:
            right = mid - 1
    return None

递归实现：

def HalfSearch(OrderedList, key, left , right):
    if left > right:
        return None
    mid = (left + right) // 2
    if key == OrderedList[mid]:
        return mid
    elif key > OrderedList[mid]:
        return HalfSearch(OrderedList, key, mid + 1, right)
    else:
        return HalfSearch(OrderedList, key, left, mid - 1)