找工作刷题记录_021二分查找

二分查找要求：线性表是有序表，即表中结点按关键字有序，并且要用向量作为表的存储结构。不妨设有序表是递增有序的。

思路：循环实现和递归实现。

#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)