算法入门（三）列表排序-binary_search

def binary_search(li, val):
    left = 0
    right = len(li) - 1
    while left <= right:  # 候选区有值
        mid = (right + left) // 2
        if li[mid] == val:
            return mid
        elif li[mid] > val:  # 查找得值 在mid的左侧
            right = mid - 1
        else:
            left = mid + 1 # 查找得值在mid 的右侧
    return None