二分查找

def binary_search(list, item):
    low = 0
    high = len(list) - 1

    while low <= high:   # 结束条件
        mid = (low + high) / 2  # 就检查中间的元素
        guess = list[mid]
        
        if guess == item:
            return mid
        if guess > item:  # 猜的数字大了
            high = mid - 1
        else:
            low = mid + 1

    return None