[2021 Spring] CS61A Discussion 5: Python Lists, Trees, Mutability
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Discussion 5地址: https://inst.eecs.berkeley.edu/~cs61a/sp21/disc/disc05/#lists
目录
Lists
Q1: Closest Number
要求一行写出,先用list(zip())获得nums列表中每个数字与目标值的差值 与它们在nums中的索引,求出最小差值和索引,根据索引求closest number。
尝试用min,发现只比较了每个元组的第一个元素,所以索引值是min(lt)[1].
lt = list(zip([abs(num - target) for num in nums], range(len(nums))))
index_min = min(lt)[1]
result = nums[index_min]
# Q1: Closest Number
def closest_number(nums, target):
"""
>>> closest_number([1, 4, 5, 6, 7], 2)
1
>>> closest_number([3, 1, 5, 6, 13], 4) # choose the earlier number since there\'s a tie
3
>>> closest_number([34, 102, 8, 5, 23], 25)
23
"""
return nums[min(list(zip([abs(num - target) for num in nums], range(len(nums)))))[1]]
Q2: (Tutorial) Max Product
# Q2: (Tutorial) Max Product
def max_product(s):
"""Return the maximum product that can be formed using non-consecutive
elements of s.
>>> max_product([10,3,1,9,2]) # 10 * 9
90
>>> max_product([5,10,5,10,5]) # 5 * 5 * 5
125
>>> max_product([])
1
"""
"*** YOUR CODE HERE ***"
if s == []:
return 1
return max(max_product(s[1:]), s[0] * max_product(s[2:]))
Mutability
Q4: (Optional) Mystery Reverse Environment Diagram
def mystery(p, q):
p[1].extend(q)
q.append(p[1:])
p = [2, 3]
q = [4, [p]]
mystery(q, p)
Q5: (Tutorial) Add This Many
# Q5: (Tutorial) Add This Many
def add_this_many(x, el, s):
""" Adds el to the end of s the number of times x occurs
in s.
>>> s = [1, 2, 4, 2, 1]
>>> add_this_many(1, 5, s)
>>> s
[1, 2, 4, 2, 1, 5, 5]
>>> add_this_many(2, 2, s)
>>> s
[1, 2, 4, 2, 1, 5, 5, 2, 2]
"""
"*** YOUR CODE HERE ***"
for i in s[:]:
if i == x:
s.append(el)
Trees
Q6: (Warmup) Height
# Q6: (Warmup) Height
def height(t):
"""Return the height of a tree.
>>> t = tree(3, [tree(5, [tree(1)]), tree(2)])
>>> height(t)
2
"""
"*** YOUR CODE HERE ***"
if not children(t):
return 1
else:
return max([height(children(c)) for c in children(t)]) + 1
Q7: Maximum Path Sum
# Q7: Maximum Path Sum
def max_path_sum(t):
"""Return the maximum path sum of the tree.
>>> t = tree(1, [tree(5, [tree(1), tree(3)]), tree(10)])
>>> max_path_sum(t)
11
"""
"*** YOUR CODE HERE ***"
if is_leaf(t):
return label(t)
else:
return max([max_path_sum(c) for c in children(t)]) + label(t)
Q8: (Tutorial) Find Path
Tree - Implementation A
def tree(label, children=[]):
return [label] + children
def label(tree):
return tree[0]
def children(tree):
return tree[1:]
def is_leaf(tree):
return len(children(tree)) == 0
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