递归函数

Posted 2021-03-08 qi1113

递归函数
    了解什么是递归  ： 在函数中调用自身函数
        最大递归深度默认是997/998 —— 是python从内存角度出发做得限制

　　递归的缺点 ： 占内存
　　递归的优点：  会让代码变简单

# alex 多大       n = 1   age(1) = age(2)+2 = age(n+1) + 2
# alex比egon大两岁
# egon多大？      n = 2   age(2) = age(3) + 2 = age(n+1) +2
# egon比wusir大两岁
# wusir多大       n = 3   age(3) = age(4) + 2 = age(n+1) +2
# wusir比金老板大两岁
# 金老板多大？
# 金老板40了      n = 4   age(4) = 40

# n = 4 age(4) = 40
# n <4  age(n) = age(n+1) +2
def age(n):
    if n == 4:
        return 40
    elif n >0 and n < 4:
        age(n+1) + 2
#
print(age(1))

# # 教你看递归
# def age(1):
#     if 1 == 4:
#         return 40
#     elif 1 > 0 and 1 < 4:
#         return 46
#
# def age(2):
#     if 2 == 4:
#         return 40
#     elif 2 >0 and 2 < 4:
#         age(3) + 2    None +2
#
# def age(3):
#     if 3 == 4:
#         return 40
#     elif 3 >0 and 3 < 4:
#         42
#
# def age(4):
#     if 4 == 4:
#         return 40
#     elif n >0 and n < 4:
#         age(n+1) + 2

二分查找算法

了解基础的算法 才能创造出更好的算法
不是所有的事情都能套用现成的方法解决的
有些时候会用到学过的算法知识来解决新的问题

# 二分查找算法 必须处理有序的列表
# l = [2,3,5,10,15,16,18,22,26,30,32,35,41,42,43,55,56,66,67,69,72,76,82,83,88]
# 5000000  4999998
# 代码实现
# def find(l,aim):
#     mid_index = len(l) // 2
#     if l[mid_index] < aim:
#         new_l = l[mid_index+1 :]
#         find(new_l,aim)
#     elif l[mid_index] > aim:
#         new_l = l[:mid_index]
#         find(new_l, aim)
#     else:
#         print(‘找到了‘,mid_index,l[mid_index])
#
# find(l,66)

l = [2,3,5,10,15,16,18,22,26,30,32,35,41,42,43,55,56,66,67,69,72,76,82,83,88]
# def find(l,aim,start = 0,end = None):
#     end = len(l) if end is None else end   # end = len(l)   24
#     mid_index = (end - start)//2 + start   #计算中间值  12 + 0 = 12
#     if l[mid_index] < aim:       #l[12] < 44   #41 < 44
#         find(l,aim,start =mid_index+1,end=end)  # find(l,44,start=13,end=24)
#     elif l[mid_index] > aim:
#         find(l, aim, start=start, end=mid_index-1)
#     else:
#         print(‘找到了‘,mid_index,aim)
#
# def find(l,aim,start = 0,end = None):    # l,44,start=13,end=24
#     end = len(l) if end is None else end   # end = 24
#     mid_index = (end - start)//2 + start   #计算中间值  24-13/2 = 5 + 13 = 18
#     if l[mid_index] < aim:       #l[18] < 44   #67 < 44
#         find(l,aim,start =mid_index+1,end=end)
#     elif l[mid_index] > aim:     # 67 > 44
#         find(l, aim, start=start, end=mid_index-1)  # find(l,44,start=13,end=17)
#     else:
#         print(‘找到了‘,mid_index,aim)
#
# def find(l,aim,start = 0,end = None):    # l,44,start=13,end=17
#     end = len(l) if end is None else end   # end = 17
#     mid_index = (end - start)//2 + start   #计算中间值  17-13/2 = 2 + 13 = 15
#     if l[mid_index] < aim:       #l[15] < 44   #55 < 44
#         find(l,aim,start =mid_index+1,end=end)
#     elif l[mid_index] > aim:     # 55 > 44
#         find(l, aim, start=start, end=mid_index-1)  # find(l,44,start=13,end=14)
#     else:
#         print(‘找到了‘,mid_index,aim)
#
# def find(l,aim,start = 0,end = None):    # l,44,start=13,end=14
#     end = len(l) if end is None else end   # end = 14
#     mid_index = (end - start)//2 + start   #计算中间值  14-13/2 = 0+ 13 = 13
#     if l[mid_index] < aim:       #l[13] < 44   #42 < 44
#         find(l,aim,start =mid_index+1,end=end)  # find(l,44,start=14,end=14)
#     elif l[mid_index] > aim:     # 42 > 44
#         find(l, aim, start=start, end=mid_index-1)
#     else:
#         print(‘找到了‘,mid_index,aim)

def find(l,aim,start = 0,end = None):
    end = len(l) if end is None else end
    mid_index = (end - start)//2 + start
    if start <= end:
        if l[mid_index] < aim:
            return find(l,aim,start =mid_index+1,end=end)
        elif l[mid_index] > aim:
            return find(l, aim, start=start, end=mid_index-1)
        else:
            return mid_index
    else:
        return ‘找不到这个值‘


ret= find(l,44)
print(ret)