網址 https://inst.eecs.berkeley.edu/~cs61a/fa21/hw/hw06/
problem1：
沒什麼難度就是看清楚邏輯照著寫就好了

    class VendingMachine:
        """A vending machine that vends some product for some price.

        >>> v = VendingMachine('candy', 10)
        >>> v.vend()
        'Nothing left to vend. Please restock.'
        >>> v.add_funds(15)
        'Nothing left to vend. Please restock. Here is your $15.'
        >>> v.restock(2)
        'Current candy stock: 2'
        >>> v.vend()
        'You must add $10 more funds.'
        >>> v.add_funds(7)
        'Current balance: $7'
        >>> v.vend()
        'You must add $3 more funds.'
        >>> v.add_funds(5)
        'Current balance: $12'
        >>> v.vend()
        'Here is your candy and $2 change.'
        >>> v.add_funds(10)
        'Current balance: $10'
        >>> v.vend()
        'Here is your candy.'
        >>> v.add_funds(15)
        'Nothing left to vend. Please restock. Here is your $15.'

        >>> w = VendingMachine('soda', 2)
        >>> w.restock(3)
        'Current soda stock: 3'
        >>> w.restock(3)
        'Current soda stock: 6'
        >>> w.add_funds(2)
        'Current balance: $2'
        >>> w.vend()
        'Here is your soda.'
        """
        "*** YOUR CODE HERE ***"
        def __init__(self, name, price):
            self.name = name 
            self.price = price
            self.stock = 0
            self.CurBal = 0
        def vend(self):
            if self.stock > 0:
                if self.price > self.CurBal:
                    print(f"'You must add ${self.price - self.CurBal} more funds.'")
                elif self.price < self.CurBal:
                    print(f"'Here is your {self.name} and ${self.CurBal - self.price } change.'")
                    self.CurBal = 0
                    self.stock -= 1
                else :
                    print(f"'Here is your {self.name}.'")
                    self.CurBal = 0
                    self.stock -= 1
            else :
                print(f"'Nothing left to vend. Please restock.'")
        def restock(self, num):
            self.stock += num
            print(f"'Current {self.name} stock: {self.stock}'")
        def add_funds(self, num):
            if self.stock <= 0:
                print(f"'Nothing left to vend. Please restock. Here is your ${num}.'")
                return 
            self.CurBal += num
            print(f"'Current balance: ${self.CurBal}'")   
 

problem2：
在子類中的屬性可以在父類提前使用（?)

    class Mint:
        """A mint creates coins by stamping on years.

        The update method sets the mint's stamp to Mint.present_year.

        >>> mint = Mint()
        >>> mint.year
        2021
        >>> dime = mint.create(Dime)
        >>> dime.year
        2021
        >>> Mint.present_year = 2101  # Time passes
        >>> nickel = mint.create(Nickel)
        >>> nickel.year     # The mint has not updated its stamp yet
        2021
        >>> nickel.worth()  # 5 cents + (80 - 50 years)
        35
        >>> mint.update()   # The mint's year is updated to 2101
        >>> Mint.present_year = 2176     # More time passes
        >>> mint.create(Dime).worth()    # 10 cents + (75 - 50 years)
        35
        >>> Mint().create(Dime).worth()  # A new mint has the current year
        10
        >>> dime.worth()     # 10 cents + (155 - 50 years)
        115
        >>> Dime.cents = 20  # Upgrade all dimes!
        >>> dime.worth()     # 20 cents + (155 - 50 years)
        125
        """
        present_year = 2021
        def __init__(self):
            self.update()
            
        def create(self, kind):
            "*** YOUR CODE HERE ***"
            return kind(self.year)

        def update(self):
            "*** YOUR CODE HERE ***"
            self.year = Mint.present_year

    class Coin:
        def __init__(self, year):
            self.year = year

        def worth(self):
            "*** YOUR CODE HERE ***"
            return self.cents + max(0,Mint.present_year - self.year - 50)

    class Nickel(Coin):
        cents = 5


    class Dime(Coin):
        cents = 10
 

problem3:

    def store_digits(n):
        """Stores the digits of a positive number n in a linked list.

        >>> s = store_digits(1)
        >>> s
        Link(1)
        >>> store_digits(2345)
        Link(2, Link(3, Link(4, Link(5))))
        >>> store_digits(876)
        Link(8, Link(7, Link(6)))
        >>> # a check for restricted functions
        >>> import inspect, re
        >>> cleaned = re.sub(r"#.*\\n", '', re.sub(r'"{3}[\s\S]*?"{3}', '', inspect.getsource(store_digits)))
        >>> print("Do not use str or reversed!") if any([r in cleaned for r in ["str", "reversed"]]) else None
        >>> link1 = Link(3, Link(Link(4), Link(5, Link(6))))
        """
        "*** YOUR CODE HERE ***"
        result = Link.empty
        while n > 0:
            result = Link(n%10, result)
            n = n // 10
        return result
 

problem4:
不好理解的的話可以暫時先理解為列表

    def deep_map_mut(fn, link):
        """Mutates a deep link by replacing each item found with the
        result of calling fn on the item.  Does NOT create new Links (so
        no use of Link's constructor)

        Does not return the modified Link object.

        >>> link1 = Link(3, Link(Link(4), Link(5, Link(6))))
        >>> # Disallow the use of making new Links before calling deep_map_mut
        >>> Link.__init__, hold = lambda *args: print("Do not create any new Links."), Link.__init__
        >>> try:
        ...     deep_map_mut(lambda x: x * x, link1)
        ... finally:
        ...     Link.__init__ = hold
        >>> print(link1)
        <9 <16> 25 36>
        """
        "*** YOUR CODE HERE ***"
        if link is Link.empty:
            return
        elif isinstance(link.first, Link):
            deep_map_mut(fn, link.first) 
        else:
            link.first = fn(link.first)
        deep_map_mut(fn, link.rest)

problem5：

    def two_list(vals, amounts):
        """
        Returns a linked list according to the two lists that were passed in. Assume
        vals and amounts are the same size. Elements in vals represent the value, and the
        corresponding element in amounts represents the number of this value desired in the
        final linked list. Assume all elements in amounts are greater than 0. Assume both
        lists have at least one element.

        >>> a = [1, 3, 2]
        >>> b = [1, 1, 1]
        >>> c = two_list(a, b)
        >>> c
        Link(1, Link(3, Link(2)))
        >>> a = [1, 3, 2]
        >>> b = [2, 2, 1]
        >>> c = two_list(a, b)
        >>> c
        Link(1, Link(1, Link(3, Link(3, Link(2)))))
        """
        "*** YOUR CODE HERE ***"
        vals.reverse()
        amounts.reverse()
        result = Link.empty
        for i in range(len(vals)):
            for j in range(amounts[i]):
                result = Link(vals[i], result)
        return result

peoblem6:
可以理解為每個class有兩個屬性，一個是本身的值，一個前一個值這樣就根據自己就可以計算出下一個數字

    class VirFib():
        """A Virahanka Fibonacci number.

        >>> start = VirFib()
        >>> start
        VirFib object, value 0
        >>> start.next()
        VirFib object, value 1
        >>> start.next().next()
        VirFib object, value 1
        >>> start.next().next().next()
        VirFib object, value 2
        >>> start.next().next().next().next()
        VirFib object, value 3
        >>> start.next().next().next().next().next()
        VirFib object, value 5
        >>> start.next().next().next().next().next().next()
        VirFib object, value 8
        >>> start.next().next().next().next().next().next() # Ensure start isn't changed
        VirFib object, value 8
        """

        def __init__(self, value=0):
            self.value = value
            self.previous = 0
        def next(self):
            if self.value == 0:
                result = VirFib(1)
            else:
                result = VirFib(self.value + self.previous)
                result.previous = self.value
            return result
        def __repr__(self):
            return "VirFib object, value " + str(self.value)

problem7：
要注意每次都要帶著一個最大值或者最小值，控制上限或者下限。

    def is_bst(t):
        """Returns True if the Tree t has the structure of a valid BST.

        >>> t1 = Tree(6, [Tree(2, [Tree(1), Tree(4)]), Tree(7, [Tree(7), Tree(8)])])
        >>> is_bst(t1)
        True
        >>> t2 = Tree(8, [Tree(2, [Tree(9), Tree(1)]), Tree(3, [Tree(6)]), Tree(5)])
        >>> is_bst(t2)
        False
        >>> t3 = Tree(6, [Tree(2, [Tree(4), Tree(1)]), Tree(7, [Tree(7), Tree(8)])])
        >>> is_bst(t3)
        False
        >>> t4 = Tree(1, [Tree(2, [Tree(3, [Tree(4)])])])
        >>> is_bst(t4)
        True
        >>> t5 = Tree(1, [Tree(0, [Tree(-1, [Tree(-2)])])])
        >>> is_bst(t5)
        True
        >>> t6 = Tree(1, [Tree(4, [Tree(2, [Tree(3)])])])
        >>> is_bst(t6)
        True
        >>> t7 = Tree(2, [Tree(1, [Tree(5)]), Tree(4)])
        >>> is_bst(t7)
        False
        """
        "*** YOUR CODE HERE ***"
        def func(tree, max, min):
            if tree.is_leaf() :
                if tree.label <= max and tree.label > min:
                    return True
                else :
                    return False
            elif len(tree.branches) == 1:  
                if tree.label <= max and tree.label > min and tree.branches[0].label <= max and tree.branches[0].label  > min:
                    return True
                else :
                    return False
            elif tree.label <= max and tree.label > min :
                return func(tree.branches[0], tree.label, -float('inf')) and func(tree.branches[1], float('inf'), tree.label)
            else :
                return False
        return func(t, float('inf'), -float('inf'))