def binary_search(lst, target):
    if len(lst) == 0:
        return None
    low, high = 0, len(lst)-1
    while low < high:
         mid = (low + high + 1) // 2 # 向上取整
         if lst[mid] <= target:
            low = mid
         else:
            high = mid - 1
    return low if lst[low] == target else None