写个函数，找出一个字符串数组中所有字符串的最长公共前缀。举例：
1、strs=｛｝，返回“”；
2、strs=｛“a”，“b”，“c”｝，返回“”；
3、strs=｛“ab”，“abc”，“abcc”｝，返回“ab”。
思路1：、以字符串0为参考，依次检查其他字符串相同位置 i （i从0到len(strs[0]遍历）是否是同一个字符，i 是递增的，当不同或者i 的值大于或等于其他字符串长度len(strs[j])（j 从 1到len(strs) 遍历，）时，返回当前的前缀。
class Solution(object):
def longestCommonPrefix(self, strs):
"""
:type strs: List[str]
:rtype: str
"""
if not strs:
return ""
res = ""
for i in range(0,len(strs[0])):
# print 'i',i
# print 'len(strs[0])',len(strs[0])
for j in xrange(1, len(strs)):
# print 'len(strs)',len(strs)
# print '######'
# print j
if i >= len(strs[j]) or strs[j][i] != strs[0][i]:
return res
res += strs[0][i]
# print '-------------------------------'
return res
#strs=['abcd','abc','abcd']
#aaa=Solution()
#str=aaa.longestCommonPrefix(strs)
#print "str: ", str方法2：将字符串数组sort排序，然后比较第一个和最后一个公共的前缀即可。
class Solution(object):
def longestCommonPrefix(self, strs):
"""
:type strs: List[str]
:rtype: str
"""
if not strs:
return ''
strs.sort()
res = ''
for i in xrange(len(strs[0])):
if i >= len(strs[-1]) or strs[-1][i] != strs[0][i]:
return res
res += strs[0][i]
return res
Sorting HOW TO
Python lists have a built-in list.sort() method that modifies the list in-place. There is also asorted() built-in function that builds a newsorted list from an iterable.
In this document, we explore the various techniques for sorting data using Python.
Sorting Basics
A simple ascending sort is very easy: just call thesorted() function. It returns a newsorted list:
>>> sorted([5, 2, 3, 1, 4])
[1, 2, 3, 4, 5]
You can also use the list.sort() method of a list. It modifies the list in-place (and returnsNone to avoid confusion). Usually it’s less convenient thansorted() - but if you don’t need the original list, it’s slightly more efficient.
>>> a = [5, 2, 3, 1, 4]
>>> a.sort()
>>> a
[1, 2, 3, 4, 5]
Another difference is that the list.sort() method is only defined for lists. In contrast, thesorted() function accepts any iterable.
>>> sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'})
[1, 2, 3, 4, 5]
Key Functions
Starting with Python 2.4, both list.sort() andsorted() added akey parameter to specify a function to be called on each list element prior to making comparisons.
For example, here’s a case-insensitive string comparison:
>>> sorted("This is a test string from Andrew".split(), key=str.lower)
['a', 'Andrew', 'from', 'is', 'string', 'test', 'This']
The value of the key parameter should be a function that takes a single argument and returns a key to use forsorting purposes. This technique is fast because the key function is called exactly once for each input record.
A common pattern is to sort complex objects using some of the object’s indices as keys. For example:
>>> student_tuples = [
... ('john', 'A', 15),
... ('jane', 'B', 12),
... ('dave', 'B', 10),
... ]
>>> sorted(student_tuples, key=lambda student: student[2]) # sort by age
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
The same technique works for objects with named attributes. For example:
>>> class Student:
... def __init__(self, name, grade, age):
... self.name = name
... self.grade = grade
... self.age = age
... def __repr__(self):
... return repr((self.name, self.grade, self.age))
>>> student_objects = [
... Student('john', 'A', 15),
... Student('jane', 'B', 12),
... Student('dave', 'B', 10),
... ]
>>> sorted(student_objects, key=lambda student: student.age) # sort by age
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
Operator Module Functions
The key-function patterns shown above are very common, so Python provides convenience functions to make accessor functions easier and faster. The operator module hasoperator.itemgetter(), operator.attrgetter(), and starting in Python 2.5 anoperator.methodcaller() function.
Using those functions, the above examples become simpler and faster:
>>> from operator import itemgetter, attrgetter
>>> sorted(student_tuples, key=itemgetter(2))
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
>>> sorted(student_objects, key=attrgetter('age'))
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
The operator module functions allow multiple levels of sorting. For example, to sort by grade then by age:
>>> sorted(student_tuples, key=itemgetter(1,2))
[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]
>>> sorted(student_objects, key=attrgetter('grade', 'age'))
[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]
The operator.methodcaller() function makes method calls with fixed parameters for each object beingsorted. For example, thestr.count() method could be used to compute message priority by counting the number of exclamation marks in a message:
>>> from operator import methodcaller
>>> messages = ['critical!!!', 'hurry!', 'standby', 'immediate!!']
>>> sorted(messages, key=methodcaller('count', '!'))
['standby', 'hurry!', 'immediate!!', 'critical!!!']
Ascending and Descending
Both list.sort() andsorted() accept areverse parameter with a boolean value. This is used to flag descending sorts. For example, to get the student data in reverse age order:
>>> sorted(student_tuples, key=itemgetter(2), reverse=True)
[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
>>> sorted(student_objects, key=attrgetter('age'), reverse=True)
[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
Sort Stability and ComplexSorts
Starting with Python 2.2, sorts are guaranteed to be stable. That means that when multiple records have the same key, their original order is preserved.
>>> data = [('red', 1), ('blue', 1), ('red', 2), ('blue', 2)]
>>> sorted(data, key=itemgetter(0))
[('blue', 1), ('blue', 2), ('red', 1), ('red', 2)]
Notice how the two records for blue retain their original order so that('blue', 1) is guaranteed to precede ('blue',2).
This wonderful property lets you build complex sorts in a series of sorting steps. For example, to sort the student data by descending grade and then ascending age, do the age sort first and then sort again using grade:
>>> s = sorted(student_objects, key=attrgetter('age')) # sort on secondary key
>>> sorted(s, key=attrgetter('grade'), reverse=True) # now sort on primary key, descending
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
The Timsort algorithm used in Python does multiple sorts efficiently because it can take advantage of any ordering already present in a dataset.
The Old Way Using Decorate-Sort-Undecorate
This idiom is called Decorate-Sort-Undecorate after its three steps:
- First, the initial list is decorated with new values that control the sort order.
- Second, the decorated list is sorted.
- Finally, the decorations are removed, creating a list that contains only the initial values in the new order.
For example, to sort the student data bygrade using the DSU approach:
>>> decorated = [(student.grade, i, student) for i, student in enumerate(student_objects)]
>>> decorated.sort()
>>> [student for grade, i, student in decorated] # undecorate
[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
This idiom works because tuples are compared lexicographically; the first items are compared; if they are the same then the second items are compared, and so on.
It is not strictly necessary in all cases to include the index i in the decorated list, but including it gives two benefits:
- The sort is stable – if two items have the same key, their order will be preserved in thesorted list.
- The original items do not have to be comparable because the ordering of the decorated tuples will be determined by at most the first two items. So for example the original list could contain complex numbers which cannot besorted directly.
Another name for this idiom is Schwartzian transform, after Randal L. Schwartz, who popularized it among Perl programmers.
For large lists and lists where the comparison information is expensive to calculate, and Python versions before 2.4, DSU is likely to be the fastest way tosort the list. For 2.4 and later, key functions provide the same functionality.
The Old Way Using the cmp Parameter
Many constructs given in this HOWTO assume Python 2.4 or later. Before that, there was nosorted() builtin andlist.sort() took no keyword arguments. Instead, all of the Py2.x versions supported acmp parameter to handle user specified comparison functions.
In Python 3, the cmp parameter was removed entirely (as part of a larger effort to simplify and unify the language, eliminating the conflict between rich comparisons and the__cmp__() magic method).
In Python 2, sort() allowed an optional function which can be called for doing the comparisons. That function should take two arguments to be compared and then return a negative value for less-than, return zero if they are equal, or return a positive value for greater-than. For example, we can do:
>>> def numeric_compare(x, y):
... return x - y
>>> sorted([5, 2, 4, 1, 3], cmp=numeric_compare)
[1, 2, 3, 4, 5]
Or you can reverse the order of comparison with:
>>> def reverse_numeric(x, y):
... return y - x
>>> sorted([5, 2, 4, 1, 3], cmp=reverse_numeric)
[5, 4, 3, 2, 1]
When porting code from Python 2.x to 3.x, the situation can arise when you have the user supplying a comparison function and you need to convert that to a key function. The following wrapper makes that easy to do:
def cmp_to_key(mycmp):
'Convert a cmp= function into a key= function'
class K(object):
def __init__(self, obj, *args):
self.obj = obj
def __lt__(self, other):
return mycmp(self.obj, other.obj) < 0
def __gt__(self, other):
return mycmp(self.obj, other.obj) > 0
def __eq__(self, other):
return mycmp(self.obj, other.obj) == 0
def __le__(self, other):
return mycmp(self.obj, other.obj) <= 0
def __ge__(self, other):
return mycmp(self.obj, other.obj) >= 0
def __ne__(self, other):
return mycmp(self.obj, other.obj) != 0
return K
To convert to a key function, just wrap the old comparison function:
>>> sorted([5, 2, 4, 1, 3], key=cmp_to_key(reverse_numeric))
[5, 4, 3, 2, 1]
In Python 2.7, the functools.cmp_to_key() function was added to the functools module.
Odd and Ends
-
For locale aware sorting, use
locale.strxfrm()for a key function orlocale.strcoll()for a comparison function. -
The reverse parameter still maintains sort stability (so that records with equal keys retain their original order). Interestingly, that effect can be simulated without the parameter by using the builtin
reversed()function twice:>>> data = [('red', 1), ('blue', 1), ('red', 2), ('blue', 2)] >>> standard_way = sorted(data, key=itemgetter(0), reverse=True) >>> double_reversed = list(reversed(sorted(reversed(data), key=itemgetter(0)))) >>> assert standard_way == double_reversed >>> standard_way [('red', 1), ('red', 2), ('blue', 1), ('blue', 2)]
-
To create a standard sort order for a class, just add the appropriate rich comparison methods:
>>> Student.__eq__ = lambda self, other: self.age == other.age >>> Student.__ne__ = lambda self, other: self.age != other.age >>> Student.__lt__ = lambda self, other: self.age < other.age >>> Student.__le__ = lambda self, other: self.age <= other.age >>> Student.__gt__ = lambda self, other: self.age > other.age >>> Student.__ge__ = lambda self, other: self.age >= other.age >>> sorted(student_objects) [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
For general purpose comparisons, the recommended approach is to define all six rich comparison operators. The
functools.total_ordering()class decorator makes this easy to implement. -
Key functions need not depend directly on the objects being sorted. A key function can also access external resources. For instance, if the student grades are stored in a dictionary, they can be used tosort a separate list of student names:
>>> students = ['dave', 'john', 'jane'] >>> grades = {'john': 'F', 'jane':'A', 'dave': 'C'} >>> sorted(students, key=grades.__getitem__) ['jane', 'dave', 'john']