Python基础14_递归函数,二分查找

 一. 递归
    在函数中调用函数本身, 就是递归
    prthon中递归的最大深度是998
        def func(n):
            print(n)
            n += 1
            func(n)
        func(1)
    递归的应用:我们可以使用递归来遍历各种树形结构, 比如我们的文件夹系统, 可以使用递归来遍历该文件夹中的所有文件
        import os

        def func(lujing, n):
        lst = os.listdir(lujing)                # os.listdir() 打开文件夹, 把该文件夹内所有的文件名装到列表lst
        for el in lst:                          # 遍历列表, 拿到每一个文件名
            path = os.path.join(lujing, el)     # 还原 文件名 的 路径
            # path = lujing + "\" + el
            # print(path)
            if os.path.isdir(path):             # 判断该路径下的文件是否是 文件夹
                print(" " * n, el)             # 打印文件名
                func(path, n+1)                 # 如果是文件夹, 再次 执行func函数, 打开该文件夹
            else:
                print(" " * n, el)         # 如果不是文件夹, 直接打印文件名
        func("d:红蜘蛛", 0)
二. 二分查找
    每次查找能删除一般的数据, 查找效率很高, 但是局限性比较大, 必须是有序序列才可以使用二分查找
    1. 普通二分法查找
        lst = [3, 15, 26, 37, 48, 59, 61, 76, 89, 92]
        def binarysearch(n, lst):
            min = 0
            max = len(lst) - 1
            while min <= max:
                mid = (min + max) // 2
                if n > lst[mid]:
                    min = mid + 1
                elif n < lst[mid]:
                    max = mid - 1
                else:
                    return mid
            else:
                return -1
        num = int(input("请输入一个数:"))
        f = binarysearch(num, lst)
        print(f)

    2. 另类递归的二分法, 很难计算出索引, 列表在变,用切片来切列表
        lst = [3, 15, 26, 37, 48, 59, 61, 76, 89, 92]
        def binarysearch(n, lst):
            min = 0
            max = len(lst) - 1
            mid = (min + max) // 2
            while lst != []:
                if n > lst[mid]:
                    lst = lst[mid + 1 :]
                    return binarysearch(n, lst)
                elif n < lst[mid]:
                    lst = lst[: mid]                    # 左闭右开
                    return binarysearch(n, lst)
                else:
                    print("存在")
                    break
            else:
                print("不存在")
        num = int(input("请输入要查找的数字:"))
        binarysearch(num, lst)
    3. 普通递归二分法, 计算思想和二分法一致
        lst = [3, 15, 26, 37, 48, 59, 61, 76, 89, 92]
        def binarysearch(n, lst, min, max):
            mid = (min + max) // 2
            if min <= max:
                if n > lst[mid]:
                    min = mid + 1
                    return binarysearch(n, lst, min, max)
                elif n < lst[mid]:
                    max = mid - 1
                    return binarysearch(n, lst, min, max)
                else:
                    return mid
            else:
                return -1
        num = int(input("请输入要查找的数:"))
        f = binarysearch(num, lst, 0, len(lst) - 1)
        print(f)
    4. 一种特殊的查找方法
        已知列表中的最大数值, 查找某个数是否存在
        lst = [3, 15, 26, 37, 48, 59, 61, 76, 89, 92]
        def func():
        while 1:
            n = int(input("请输入要查找的数字:"))
            if n <= 92 and n >= -93:
                new_list = []
                for i in range(93):
                    new_list.append(0)
                for i in range(len(lst)):
                    new_list[lst[i]] = 1
                if new_list[n] == 1:
                    print("存在")
                else:
                    print("不存在")
            else:
                print("输入有误")
                continue
        func()