2021 fall lab8

网址 https://inst.eecs.berkeley.edu/~cs61a/fa21/lab/lab08/
problem1：

problem2:

    def convert_link(link):
        """Takes a linked list and returns a Python list with the same elements.

        >>> link = Link(1, Link(2, Link(3, Link(4))))
        >>> convert_link(link)
        [1, 2, 3, 4]
        >>> convert_link(Link.empty)
        []
        """
        "*** YOUR CODE HERE ***"
        ans = []
        while link is not Link.empty:
          ans.append(link.first)
          link = link.rest
        return ans

problem3:

problem4：

    def label_squarer(t):
        """Mutates a Tree t by squaring all its elements.

        >>> t = Tree(1, [Tree(3, [Tree(5)]), Tree(7)])
        >>> label_squarer(t)
        >>> t
        Tree(1, [Tree(9, [Tree(25)]), Tree(49)])
        """
        "*** YOUR CODE HERE ***"
        t.label = t.label * t.label
        for i in t.branches:
          label_squarer(i)

problem5：

    def cumulative_mul(t):
        """Mutates t so that each node's label becomes the product of all labels in
        the corresponding subtree rooted at t.

        >>> t = Tree(1, [Tree(3, [Tree(5)]), Tree(7)])
        >>> cumulative_mul(t)
        >>> t
        Tree(105, [Tree(15, [Tree(5)]), Tree(7)])
        """
        "*** YOUR CODE HERE ***"
        for i in t.branches:
          cumulative_mul(i)
          t.label = t.label * i.label

problem6:
先从子树开始加v，然后往上，这样就不会使得v被认为是leaf

    def add_d_leaves(t, v):
        """Add d leaves containing v to each node at every depth d.
        "*** YOUR CODE HERE ***"
        def depth(t, v, d):
          for i in t.branches:
            depth(i, v, d+1)
          for _ in range(d):
            t.branches.append(Tree(v))
        return depth(t, v, 0)

problem7：

    def every_other(s):
        """Mutates a linked list so that all the odd-indiced elements are removed
        (using 0-based indexing).

        >>> s = Link(1, Link(2, Link(3, Link(4))))
        >>> every_other(s)
        >>> s
        Link(1, Link(3))
        >>> odd_length = Link(5, Link(3, Link(1)))
        >>> every_other(odd_length)
        >>> odd_length
        Link(5, Link(1))
        >>> singleton = Link(4)
        >>> every_other(singleton)
        >>> singleton
        Link(4)
        """
        "*** YOUR CODE HERE ***"
        while s != Link.empty:
          if s.rest:
            s.rest  = s.rest.rest
            s = s.rest
          else:
            s = s.rest

problem8：
从上往下，可以最大减少运行时间

    def prune_small(t, n):
        """Prune the tree mutatively, keeping only the n branches
        of each node with the smallest label.

        >>> t1 = Tree(6)
        >>> prune_small(t1, 2)
        >>> t1
        Tree(6)
        >>> t2 = Tree(6, [Tree(3), Tree(4)])
        >>> prune_small(t2, 1)
        >>> t2
        Tree(6, [Tree(3)])
        >>> t3 = Tree(6, [Tree(1), Tree(3, [Tree(1), Tree(2), Tree(3)]), Tree(5, [Tree(3), Tree(4)])])
        >>> prune_small(t3, 2)
        >>> t3
        Tree(6, [Tree(1), Tree(3, [Tree(1), Tree(2)])])
        """
        while len(t.branches) > n:
            largest = max(t.branches, key=lambda x : x.label)
            t.branches.remove(largest)
        for __ in t.branches:
            prune_small(__, n)