#實現一個二分查詢
#輸入：一個順序表list和要找的元素
#輸出：待查詢的元素的位置

def binary_search(list,item):
    low = 0
    high = len(list) - 1
    while low <= high:
        mid = (low + high) // 2
        guess = list[mid]
        if guess == item:
            return mid
        if guess < mid:
            high = mid - 1
        else:
            low = mid + 1
    return None


test = [0, 1, 2, 8, 13, 17, 19, 32, 42]
print(binary_search(test,13))