问题描述
我想在一个无序列月的列表中找到最大的集合，它可以作为不同的、连续的个月的有序列表返回。
例如：
consecutive_months(["December","January","February","April"])
会输出：
"December","February"
还有：
consecutive_months(["February","December","January"])
会输出：
"December","February"
以下有效，但我很好奇是否有人对更优雅的方式有想法：
MONTHS = ["January","march","April","May","June","July","August","September","October","November","December"]
def consecutive_months(lst_of_months):
# create two years of months to handle going from Dec to Jan
results = []
for m in set(lst_of_months):
results.append((m,MONTHS.index(m)))
results.append((m,MONTHS.index(m)+12))
results = sorted(results,key=lambda x: x[1])
# find the longest series of consecutive months
this_series = []
longest_series = []
for this_month in results:
if len(this_series) > 0:
last_month = this_series[-1]
if last_month[1] + 1 == this_month[1]:
this_series.append(this_month)
else:
this_series = [this_month]
else:
this_series = [this_month]
if len(this_series) > len(longest_series):
longest_series = [m for (m,i) in this_series]
return longest_series
解决方法
我注意到您的代码存在一个问题：当所有 12 个月都出现在输入中时，输出会列出所有月份两次。这很容易解决，只需：
return longest_series[:12]
我会寻求一种解决方案，将输入转换为一种 12 位的“位图”，其中 1 表示输入中对应的月份，而 0 表示不是。
如果表示为 12 个字符的字符串，则可以使用正则表达式轻松识别“1”的序列。
我还会对月份列表进行一些预处理，这样你就有了列表和它的字典版本，并将列表翻了一番，这样你就可以从它的 12 边界切片。
这是建议的代码：
import re
months = ["January","February","March","April","May","June","July","August","September","October","November","December"]
# Also create a dictionary to get a month's index
month_nums = { month: num for num,month in enumerate(months) }
# ... and double the months list,to ease slicing across the 12-boundary
months += months
def consecutive_months(given_months):
# Deal with boundary case
if not given_months:
return []
# Convert input to 12 bits in string format
lst = ["0"] * 12
for m in given_months:
lst[month_nums[m]] = "1"
bits = "".join(lst)
# Identify the longest chunk of consecutive "1" in that doubled string
_,start,end = max((j-i,i,j)
for i,j in (match.span(0)
for match in re.finditer("1+",bits + bits)
)
)
# Using the found span,extract the corresponding month names
return months[start:end][:12]
以下是一位也研究过该问题的朋友提供的两种可行方法。第一个是高效的并且使用模运算符，因此列表不需要复制到自身上。
month_names = [
'January','February','March','April','May','June','July','August','September','October','November','December'
]
# Looks like: {'January': 0,'February': 1...}
month_name_to_index = {
value: index
for index,value
in enumerate(month_names)
}
def consecutive_months(list_of_months_by_name):
if not list_of_months_by_name:
# If the list is empty,return None.
return None
month_was_seen = [False] * 12 # Looks like: [False,False,...]
for month_name in list_of_months_by_name:
month_was_seen[month_name_to_index[month_name]] = True
# Seek to first missing month:
for start_index in range(12):
if not month_was_seen[start_index]:
break
# If there is no missing month,return the whole year.
if month_was_seen[start_index]:
return {"from": "January","to": "December","length": 12}
# Starting from the first missing month,loop around the year
# and keep track of the longest run using one boolean and four
# integers.
running = False
longest_run_index = None
longest_run_length = 0
current_run_index = None
current_run_length = None
for offset in range(1,13):
index = (start_index + offset) % 12
if month_was_seen[index]:
# Continue a run or begin a run.
if running:
current_run_length += 1
continue
running = True
current_run_index = index
current_run_length = 1
continue
if running:
# End the run.
running = False
if current_run_length > longest_run_length:
longest_run_index = current_run_index
longest_run_length = current_run_length
return {
"from": month_names[longest_run_index],"to": month_names[(longest_run_index + longest_run_length - 1) % 12],"length": longest_run_length
}
第二个是巧妙的单线：
MONTH_NAMES = [
'January','December'
]
def consecutive_months(list_of_months_by_name):
return max(
(
len(segment)-segment.index(":")-1,(MONTH_NAMES*2)[
int(segment[:segment.index(":")])+1
:
int(segment[:segment.index(":")]) + len(segment) - segment.index(":")
]
)
for segment in
"".join([
"x" if month_name in list_of_months_by_name else f",{index}:"
for index,month_name in enumerate(MONTH_NAMES*2)
]).split(",")
if ":" in segment
)[1] if set(MONTH_NAMES) - set(list_of_months_by_name) else MONTH_NAMES
两种算法都返回测试数据的预期结果。谢谢影音！